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.     

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.     

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.     

Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO₂ acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO₂ substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO₂ occurring via the strong bonding between the conduction bands of QDs and TiO₂ is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO₂ acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO₂ systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO₂ acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.     

Development and Investigation of Ultrastable PbS/CdS/ZnS Quantum Dots for Near‐Infrared Tumor Imaging

Achieving bright, reliable, robust, and stable probes for in vivo imaging is becoming extremely urgent for the cancer imaging research community. To date very few works have reported on elucidating in the varied and chemically complex biological milieu. The authors report detailed investigations of the synthesis of near‐infrared, water dispersive, strongly luminescent, and highly stable PbS/CdS/ZnS core/shell/shell quantum dots (QDs). These QDs are extremely stable, they could keep their initial morphology, dispersion status, and photoluminescence (PL) in phosphate buffered saline buffer for as long as 14 months. The QDs also show excellent photostability and could keep ≈80% of their initial PL intensity after 1 h continuous, strong UV illumination. More interestingly, they show negligible toxicity to cultured cells even at high QDs concentration. Given these outstanding properties, the QDs are explored for in vivo, tumor imaging in mice. With one order of magnitude lower QD concentration (0.04 mg mL^–1), significantly weaker laser intensity (0.04 W cm^–2 vs ≈1 W cm^–2), and considerably shorter signal integration time (≤1 ms vs hundreds of ms) as compared to the best reported rare earth doped nanoparticles, the QDs show high emission intensity even at injection depth of ≈2.5 mm.     

Sonoelectrochemical synthesis of water-soluble CdTe quantum dots

A facile and fast one-pot method has been developed for the synthesis of CdTe quantum dots (QDs) in aqueous phase by a sonoelectrochemical route without the protection of N₂. The morphology, structure and composition of the as-prepared products were investigated by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDS). The influences of current intensity, current pulse width, and reaction temperature on the photoluminescence (PL) and quantum yield (QY) of the products were studied. The experimental results showed that the water-soluble CdTe QDs with high PL qualities can be conveniently synthesized without precursor preparation and N₂ protection, and the PL emission wavelength and QY can be effectively controlled by adjusting some parameters. This method can be expected to prepare other QDs as promising building blocks in solar cell, photocatalysis and sensors.     

2018年5期 电子期刊

研究了不同Mn/Pb量比的Mn掺杂CsPbCl₃（Mn：CsPbCl₃）钙钛矿量子点的发光性质。Mn/Pb的量比增加引起的Mn²⁺发光峰的红移,被认为是来源于高浓度Mn²⁺掺杂下的Mn²⁺-Mn²⁺对。进一步研究了Mn：CsPbCl₃量子点的发光效率与Mn/Pb的量比之间的关系,发现随着量比达到5：1时,其发光效率明显下降。这种发光效率下降是由于Mn掺杂浓度引起的发光猝灭。Mn：CsPbCl₃量子点的变温发光光谱证实,随着温度的升高,Mn离子发光峰蓝移,线宽加宽,但其发光强度明显增加。     

Robust Fabrication of Quantum Dots on Few‐Layer MoS2 by Soft Hydrogen Plasma and Post‐Annealing

Apart from unique properties of layered transition‐metal dichalcogenide nanosheets like MoS₂, quantum dots (QDs) from these layered materials promise novel science and applications due to their quantum confinement effect. However, the reported fabrication techniques for such QDs all involve the use of liquid organic solvents and the final material extraction from such liquid dispersions. Here a novel and convenient dry method for the synthesis of MoS₂ quantum dots interspersed on few‐layer MoS₂ using soft hydrogen plasma treatment followed by post‐annealing is demonstrated. The size of MoS₂ nanodots can be well controlled by adjusting the working pressure of hydrogen plasma and post‐thermal annealing. This method relies on the cumulative hydrogen ion bombardment effect which can destroy the hexagonal structure of the top MoS₂ layer and disintegrate the top layer into MoS₂ nanodots and even QDs. Post‐thermal annealing can further reduce the size. Such MoS₂ quantum dots interspersed on few‐layer MoS₂ exhibit two new photoluminescence peaks at around 575 nm because of the quantum confinement effect. This dry method is versatile, scalable, and compatible with the semiconductor manufacturing processes, and can be extended to other layered materials for applications in hydrogen evolution reaction, catalysis, and energy devices.     

Investigation of Interfacial Charge Separation at PbS QDs/(001) TiO2 Nanosheets Heterojunction Solar Cell

In the recent years, the heterojunction solar cells based on quantum dots (QDs) have attracted attention due to strong light absorbing characteristics and the size effect on the bandgap tuning. This paper reports on the kinetics of interfacial charge separation of PbS QDs/(001) TiO₂ nanosheets heterojunction solar cells. PbS QDs are deposited using a bifunctional linker molecule on two different TiO₂ films, i.e., TiO₂ nanosheets (with 001 dominant exposed facet) and TiO₂ nanoparticles (with 101 dominant exposed facet). Upon bandgap excitation, electrons are transferred from the PbS QDs conduction band to the lower lying conduction band of TiO₂. Based on the ultrafast pump‐probe laser spectroscopy technique, the kinetics of charge separation is scrutinized at the PbS/TiO₂ interface. The interfacial charge separation at PbS/TiO₂ nanosheets films made of (001) dominant exposed facets is five times faster than that on (101) dominant exposed facets TiO₂ nanoparticles. The quantum yields for charge injection are higher for the (001) TiO₂ nanosheets than the (101) TiO₂ nanoparticles due to enhanced interfacial interaction with (001) surface compared to the (101) nanoparticles. The superior interfacial charge separation at PbS/(001) nanosheets respect to PbS/(101) nanoparticles is consistent with the higher photocurrent and enhanced power conversion efficiency in the PbS QDs/(001) TiO₂ heterojunction solar cell. The use of (001) TiO₂ nanosheets can be a better alternative to conventional mesoporous TiO₂ films in QD heterojunction solar cells and perovskites‐based heterojunction solar cells.     

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。     

Preparation of Highly Stable and Photoluminescent Cadmium-Free InP/GaP/ZnS Core/Shell Quantum Dots and Application to Quantitative Immunoassay

Indium phosphide (InP) quantum dots (QDs) are ideal substitutes for widely used cadmium-based QDs and have great application prospects in biological fields due to their environmentally benign properties and human safety. However, the synthesis of InP core/shell QDs with biocompatibility, high quantum yield (QY), uniform particle size, and high stability is still a challenging subject. Herein, high quality (QY up to 72%) thick shell InP/GaP/ZnS core/shell QDs (12.8 ± 1.4 nm) are synthesized using multiple injections of shell precursor and extension of shell growth time, with GaP serving as the intermediate layer and 1-octanethiol acting as the new S source. The thick shell InP/GaP/ZnS core/shell QDs still keep high QY and photostability after transfer into water. InP/GaP/ZnS core/shell QDs as fluorescence labels to establish QD-based fluorescence-linked immunosorbent assay (QD-FLISA) for quantitative detection of C-reactive protein (CRP), and a calibration curve is established between fluorescence intensity and CRP concentrations (range: 1–800 ng mL⁻¹, correlation coefficient: R² = 0.9992). The limit of detection is 2.9 ng mL⁻¹, which increases twofold compared to previously reported cadmium-free QD-based immunoassays. Thus, InP/GaP/ZnS core/shell QDs as a great promise fluorescence labeling material, provide a new route for cadmium-free sensitive and specific immunoassays in biomedical fields.     

Atomic layer deposition of TiO2 and Al‐doped TiO2 films on Ir substrates for ultralow leakage currents

TiO₂ and Al‐doped TiO₂ (ATO) films were grown on Ir substrates by atomic layer deposition using O₃ as the oxygen source. With increasing O₃ feeding time, the crystalline structure of the TiO₂ films was transformed from anatase to rutile. Above an O₃ feeding time of 35 s, the films crystallized as only rutile due to the formation of IrO₂ layer at the interface. The TiO₂ and ATO films showed higher dielectric constants of 78 and 51, respectively. The films on Ir showed superior leakage properties compared to the films on Ru due to the high work‐function of Ir. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultrasonic enhancement of microdroplet-based interfacial reaction for improving the synthesis of Ag2S QDs

Ag₂S quantum dots (QDs) have aroused extensive concerns in intravital imaging field due to their merits of narrow bandgap, low biological toxicity and decent fluorescence emission properties in the second near-infrared (NIR-II) window. However, low quantum yield (QY) and poor uniformity of Ag₂S QDs are still main obstacles for its application. In this work, a novel strategy of utilizing ultrasonic field is presented, which can enhance the microdroplet-based interfacial synthesis of Ag₂S QDs. The ultrasound increases the presence of ions at the reaction sites by enhancing the ion mobility in the microchennels. Therefore, the QY is enhanced from 2.33 % (optimal QY without ultrasound) to 8.46 %, which is the highest value of Ag₂S ever reported without ion-doping. Also, the decrease of the corresponding full width at half maximum (FWHM) from 312 nm to 144 nm indicates the obvious uniformity improvement of the obtained QDs. Further mechanism exploration illustrates that ultrasonic cavitation significantly increases the interfacial reaction sites by splitting the droplets. Meanwhile, the acoustic flow field strengthens the ion renewal at the droplet interface. Consequently, the mass transfer coefficient increases by more than 500 %, which is favorable to improve both the QY and quality of Ag₂S QDs. This work serves both fundamental research and practical production for the synthesis of Ag₂S QDs.     

Efficient Quantum Dot Light-Emitting Diodes Based on Well-Type Thick-Shell CdxZn1−xS/CdSe/CdyZn1−yS Quantum Dots

Device grade quantum dots (QDs) require QDs ensembles to retain their original superior optical properties as in solution. QDs with thick shells are proven effective in suppressing the inter-dot interaction and preserving the emission properties for QDs solids. However, lattice strain–induced defects may form as the shell grows thicker, resulting in a notable photoluminescence quenching. Herein, a well-type Cd_xZn_1−xS/CdSe/Cd_yZn_1−yS QDs is proposed, where ternary alloys CdZnS are adopted to match the lattice parameter of intermediate CdSe by separately adjusting the x and y parameters. The resultant thick-shell Cd_0.5Zn_0.5S/CdSe/Cd_0.73Zn_0.27S QDs reveal nonblinking properties with a high PL QY of 99% in solution and 87% in film. The optimized quantum dot light-emitting diodes (QLEDs) exhibit a luminance of 31547.5 cd m⁻² at the external quantum efficiency maximum of 21.2% under a bias of 4.0 V. The shell thickness shows great impact on the degradation of the devices. The T₅₀ lifetime of the QLEDs with 11.2 nm QDs reaches 251 493 h, which is much higher than that of 6.5 and 8.4 nm QDs counterparts. The performances of the well-type thick-shell QLEDs are comparable to state-of-the-art devices, suggesting that this type of QDs is a promising candidate for efficient optoelectronic devices.     

Water‐Soluble Monolayer Molybdenum Disulfide Quantum Dots with Upconversion Fluorescence

Uniform water‐soluble monolayer MoS₂ quantum dots (MQDs) with lateral sizes of ≈2.1 nm, a clearly zigzag‐terminated edge, and a hexagonal lattice structure are achieved using ammonium molybdate, thiourea, and N‐acetyl‐l ‐cysteine (NAC) as precursors and the capping reagent in a facile one‐pot hydrothermal approach. MQDs have good dispersity and high stability in aqueous suspension and exhibit a significantly larger direct bandgap (3.96 eV) compared to monolayer MoS₂ nanosheets (1.89 eV). Pronounced blue‐shifts in the wavelengths of both the excitonic absorption and intrinsic state emission with activated strong luminescence at room temperature beyond monolayer MoS₂ nanosheets is demonstrated. Unusual upconversion photoluminescence is also observed and is caused by two successive transfers of energy from the near‐infrared (NIR) absorption generated by the NAC capping reagent to the hexagonal structure of MQDs. Additional optical properties of MQDs may provide numerous exciting technological applications. Here, MQDs are demonstrated as a highly selective fluorescent reagent for detecting tetracycline hydrochloride under UV and NIR irradiation.     

A Simple Strategy to Prepare Colloidal Cu‐Doped ZnSe(S) Green Emitter Nanocrystals in Aqueous Media

A novel and simple method is described for preparing colloidal Cu‐doped ZnSe(S) quantum dots (QDs) in aqueous media by introducing copper ions using the same method as to prepare colloidal ZnSe(S). More specifically, the Cu‐doped ZnSe(S) are prepared through the nucleation‐doping method in the presence of 3‐mercaptopropionic acid as stabilizers using zinc perchlorate, copper sulphate, and NaHSe as precursors. Confirmation of the preparation of Cu‐doped ZnSe(S) nanocrystals (NCs) is done with absorption and emission spectroscopies (UV–vis and PL) as the QDs show intensive green emissions. The reduction of ions Cu²⁺ to Cu⁺ is confirmed by using electron paramagnetic resonance (EPR), in which Cu⁺ ions are silent. The size determination is performed by using transmission electron microscopy (TEM) and dynamic light scattering (DLS), resulting in Cu‐doped ZnSe(S) particles with a mean diameter of 4.6 ± 3.5 nm. The excellent stability observed for the nanoparticles overcomes the intrinsic instability of traditional aqueous Cu‐doped ZnSe(S) NCs.     

Antireflection ln2O3 coatings of self‐organized TiO2 nanotube layers prepared by atomic layer deposition

We report on the uniform anti‐reflection coating of TiO₂ nanotube layers with a secondary material – indium trioxide (In₂O₃) – by atomic layer deposition (ALD). We provide for the first time the detailed evidence of the ALD deposited coating inside nanotubes for three different tube layers with aspect ratio up to ≈80, which is so far the highest aspect ratio reported for ALD‐processed self‐organized anodic TiO₂ nanotubes. We show that uniform In₂O₃coating of the nanotubes strongly influences the overall reflectance of the layers due to intrinsic properties of In₂O₃. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Quantum emission efficiency of nanocrystalline and amorphous Si quantum dots

The paper presents the comparison of emission efficiencies for crystalline Si quantum dots (QDs) and amorphous Si nanoclusters (QDs) embedded in hydrogenated amorphous (a-Si:H) films grown by the hot wire-CVD method (HW-CVD) at the variation of technological parameters. The correlations between the intensities of different PL bands and the volumes of Si nanocrystals (nc-Si:H) and/or an amorphous (a-Si:H) phase have been revealed using X-ray diffraction (XRD) and photoluminescence (PL) methods. These correlations permit to discuss the PL mechanisms in a-Si:H films with embedded nc-Si QDs. The QD parameters of nc-Si:H and a-Si:H QDs have been estimated from PL results and have been compared (for nc-Si QDs) with the parameters obtained by the XRD method. Using PL and XRD results the relations between quantum emission efficiencies for crystalline (η_cr) and amorphous (η_am) QDs have been estimated and discussed for all studied QD samples. It is revealed that a-Si:H films prepared by HW-CVD with the variation of wire temperatures are characterized by better passivation of nonradiative recombination centers in comparison with the films prepared at the variation of substrate temperatures or oxygen flows.     

Electrostatic assembly of CdTe quantum dots with different charged ligands into TiO2 porous film for solar cells

We have demonstrated an approach for the electrostatic assembly of CdTe quantum dots (QDs) with different charged ligands as sensitizers, achieving high coverage and good dispersion in TiO₂ porous films. The CdTe QD-sensitized TiO₂ porous films were subjected to thermal annealing in a high vacuum chamber to remove the ligand linker, resulting in the formation of direct heterojunctions between the bare CdTe QDs and TiO₂ for a favorable charge transfer. The as-received CdTe QD-sensitized TiO₂ porous films were employed as photoanodes for quantum dot-sensitized solar cells (QSSCs), and the photocurrent density reached as high as 4.69 mA/cm² under a standard illumination condition of simulated AM 1.5G (100 mW/cm²).     

Effect of TiO2 crystalline composition on the dielectric properties of TiO2/P(VDF‐TrFE) composites

In order to study the effect of the TiO₂ particle crystalline composition (with different proportions of rutile and anatase crystals) on the dielectric properties of the composite, titanium dioxide (TiO₂) particles and TiO₂/poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)] composites were synthesized by a reflux method and the solution route, respectively. The results indicated that the optimum TiO₂ particle crystalline composition is anatase content of 37% and rutile content of 63% for dielectric‐constant modifier applications. Furthermore, a dielectric constant of 25.7 with dielectric loss of 0.17 at 100 Hz at room temperature were obtained in the composite with 40 wt% TiO₂ particles. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)