Optical Properties of Core-Multishell Quantum Dots

Ternary and quaternary semiconductor quantum dots (QDs) are candidates for cadmium-free alternatives. Among these, semiconductors containing elements from groups 11, 13, and 16 (i.e., I–III–VI2) are attracting increasing attention since they are direct semiconductors whose bandgap energies in the bulk state are tunable between visible and near infrared. The quaternary system of alloys consisting of silver indium sulfide (AgInS2; bandgap energy: Eg = 1.8 eV) and silver gallium sulfide (AgGaS2; Eg = 2.4 eV) (i.e., Ag[InxGa1−x]S2 (AIGS)) enables bandgap tuning over a wide range of visible light. However, the photoluminescence (PL) quantum yield (10–20%) of AIGS QDs is significantly lower than that of AgInS2 (60–70%). The present study investigates how to improve the PL quantum yield of AIGS QDs via surface ligand engineering. Firstly, the use of a mixture of oleic acid and oleylamine, instead of only oleylamine, as the solvent for the QD synthesis was attempted, and a threefold improvement of the PL quantum yield was achieved. Subsequently, a post-synthetic ligand exchange was performed. Although the addition of alkylphosphine, which is known as an L-type ligand, improved the PL efficiency only by 20%, the use of metal halides, which are categorized as Z-type ligands, demonstrated a twofold to threefold improvement of the PL quantum yield, with the highest value reaching 73.4%. The same procedure was applied to the band-edge emitting core/shell-like QDs that were synthesized in one batch based on our previous findings. While the as-prepared core/shell-like QDs exhibited a PL quantum yield of only 9%, the PL quantum yield increased to 49.5% after treatment with metal halides.

CdSe/Cd(1-x) Zn(x)S core/shell quantum dots (QDs) were fabricated in 1-octadecene via a two step synthesis. CdSe cores were first prepared using CdO, trioctylphosphine (TOP) selenium, and stearic acid. Subsquently, a Cd(1-x) Zn(x)S shell coating was carried out using zinc acetate dihydrate, cadmium acetate dihydrate, TOPS, and hexadecylamine (HDA) starting materials in the friendly organic system under relatively low temperature. The absorption and photoluminescence (PL) spectra have a significant red shift after the coverage of Cd(1-x)Zn(x)S shell on CdSe cores. The X-ray diffraction analysis of samples confirmed the formation of core/shell structure. The PL quantum yields (QYs) of CdSe/Cd(1-x)Zn(x)S QDs were improved gradually with time at room temperature. This is ascribed to the surface passivation of HDA to the QDs during store. This phenomenon was confirmed by the Fourier transform infrared spectrum of samples. Namely, HDA does not capped on the surface of as-prepared QDs, in which a low PL QYs was observed (less than 10%). Being storing for certain time, HDA attached to the surface of the QDs, in which the PL QYs increased (up to 31%) and the full width at half maximum of PL spectra decreased. Moreover, the fluorescence decay curve of the core/shell QDs is closer to a biexponential decay profile and has a longer average PL lifetime. The variation of average PL lifetime also indicated the influence of HDA during store.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

Well-resolved oil-soluble Au-doped ZnCdS quantum dots and enhancing doping emission with In-codoping

In the blossoming field of Cd-free semiconductor quantum dots (QDs), ternary I-III-VI QDs have received increasing attention due to the ease of the environmentally friendly synthesis of high-quality materials in water, their high photoluminescence (PL) quantum yields (QYs) in the red and near infrared (NIR) region, and their inherently low toxicity. Moreover, their oxygen-insensitive long PL lifetimes of up to several hundreds of nanoseconds close a gap for applications exploiting the compound-specific parameter PL lifetime. To overcome the lack of reproducible synthetic methodologies and to enable a design-based control of their PL properties, we assessed and modelled the synthesis of high-quality MPA-capped AgInS2/ZnS (AIS/ZnS) QDs. Systematically refined parameters included reaction time, temperature, Ag:In ratio, S:In ratio, Zn:In ratio, MPA:In ratio, and pH using a design-of-experiment approach. Guidance for the optimization was provided by mathematical models developed for the application-relevant PL parameters, maximum PL wavelength, QY, and PL lifetime as well as the elemental composition in terms of Ag:In:Zn ratio. With these experimental data-based models, MPA:In and Ag:In ratios and pH values were identified as the most important synthesis parameters for PL control and an insight into the connection of these parameters could be gained. Subsequently, the experimental conditions to synthetize QDs with tunable emission and high QY were predicted. The excellent agreement between the predicted and experimentally found PL features confirmed the reliability of our methodology for the rational design of high quality AIS/ZnS QDs with defined PL features. This approach can be straightforwardly extended to other ternary and quaternary QDs and to doped QDs.

Nano-onions with multifold alternating CdS/CdSe or CdSe/CdS structure have been synthesized via a two-phase approach. The influences of shell on photoluminescence (PL) quantum yields (QYs) and PL lifetimes are investigated and discussed. It is found that the outmost shell plays an important role in the PL QYs and PL lifetimes of the multishells onion-like nanocrystals. The PL QYs and PL lifetimes fluctuate regularly with CdSe and CdS shells. The PL QY increases when the nanocrystals have an outmost CdS shell; however, it decreases dramatically with the outmost CdSe shell. The trend of the change of PL lifetimes is consistent with that of the QYs. The crystal structure and composition of the novel nano-onions are characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra techniques.

Variation of the steady-state optical properties of quantum dots (QDs) caused by photoinduced charge transfer is a widely known phenomenon which hinders QD applications requiring long-term stability of the optical signal, for example, bioimaging, single-photon sources, molecular tracking, and lasing. Intense light irradiation may provoke QD photodarkening, when their photoluminescence (PL) quantum yield (QY) is reduced, or photobrightening, when the QY increases and QD PL becomes brighter. In order to understand the nature of these processes and to determine the optimal operating conditions for QDs with different core sizes and shell thicknesses, we have systematically studied the photodarkening and photobrightening phenomena for different wavelengths and intensities of irradiation. The results have shown that changes in the core size only slightly alter the effect of light irradiation on the QD optical properties, whereas the irradiation wavelength and shell thickness are the major factors that determine the variation of the QD PL QY. We have found that the observed photoinduced processes are independent of the irradiation intensity, which proves the single-photon origin of these processes. We have also used comparative data on irradiation of QD solution and QDs embedded in polymer matrices to develop a general model describing the mechanism of the QD sensitivity to intense irradiation and elucidating the role of surface ligands in this mechanism. Our findings determine the strategy for increasing QD optical stability and optimization of operational conditions for QD-based devices.

Zero reduction luminescence of aqueous-phase alloy core/shell quantum dots via rapid ambient-condition ligand exchange

The entrapment of quantum dots (QDs) in the inner part of micelles formed by surfactant polymers is a powerful methodology to prepare stable and photoluminescent core nanoparticles with enhanced optical properties. These features are crucial for the application of QDs in the design of hybrid assemblies for light harvesting applications, where energy transfer processes are required. The present work was focused on the synthesis of a surfactant homopolymer, poly (acrylic acid) (PAA) macroRAFT, to be used as a stabilizer of hydrophobic cadmium telluride (CdTe) QDs in aqueous solution. PAA macroRAFT was synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization in a single chemical reaction. Its micelles were used to entangle and entrap hydrophobic CdTe QDs, with different molar ratio of polymer and QDs. The morphology and optical properties of the entrapped QDs were determined. The results showed that PAA macroRAFT is able to form micelles with a critical micelle concentration of 2.08 mg/mL. It was also noticed that the molar ratio of polymer and QDs have high influence on the QDs’ morphology and their optical properties. The QDs’ photoluminescence quantum yield was enhanced approximately 23% upon their entrapment in PAA macroRAFT micelles, using 60 equivalents of polymer. Moreover, while in solution, QDs are well-dispersed, having a 3.5 nm diameter, upon being entrapped in the micelles, tend to form clusters with a size around 100 nm.

Colloidal semiconductor nanocrystals, or quantum dots (QDs), show great promise as light absorbers and emitters in next-generation optoelectronic devices, but deleterious charge and energy-transfer processes that occur in solid-state thin films of QDs hinder their wide-scale utilization. Several classes of materials have been used previously to encapsulate QDs in the solid state but have failed to fully prevent charge and energy transfer in densely packed thin films of QDs relevant for device applications, necessitating the exploration of other materials. This paper describes the use of a metal–organic framework (MOF), zeolitic imidazolate framework-8 (ZIF-8), as a novel matrix encapsulation material to regulate the inter-QD spacing of core–shell QDs in a crystalline host, thus enhancing the photoluminescence quantum yield (PL QY) and irradiance of the films. The brightest composite films have PL QYs of 7.4–9.8% (whereas the PL QY of the QD-only film is 2.1%) and volume-normalized irradiances that are at least a factor of 5 higher than that of the QD-only film. Spectrally resolved PL lifetime measurements indicate that ZIF-8 improves the emission of the QDs by inhibiting inter-QD energy transfer, primarily by preventing the formation of large aggregates of QDs that are present in the film of poly(vinylpyrrolidone) (PVP)-coated QDs without ZIF-8. This demonstration of control over QD spacing within the crystalline MOF matrix reveals new pathways toward mesoscale QD structures with controlled electronic coupling and dielectric environments. Furthermore, this study is one of the first investigations of the photophysics of QD@MOF composite materials.

Core/shell-structured quantum dots (QDs) are considered as important active materials for optoelectronic devices. There have been a lot of synthesis procedures developed so far. Real epitaxial growth of shell layer, however, has not been reported yet. Here, a simple method for the synthesis of CdSe/CdS core/shell QDs is presented. Epitaxial growth of CdS shell on CdSe core is carried out by near room temperature successive ion layer adsorption and reaction (RT-SILAR). Our method for shell formation is conducted at room temperature, which facilitates the separation of resulting products from shell growth solution. After full coverage with one monolayer (ML) of CdS on CdSe QDs surface, photoluminescence (PL) quantum yield (QY) reaches up to 60%. Produced CdSe/CdS QDs have an elongated morphology, implying that CdS layers are formed in an epitaxial manner without etching or deteriorating CdSe QDs surface. We believe that our synthesis method for core/shell-structured QDs would be an ideal model for practical implication of QDs as well as fundamental studies.

Colloidal quantum dots (QDs) have become multipurpose luminophores that combine a broad excitation with a narrow emission spectrum. Applications in displays, lighting, or solar-energy conversion, however, require quantum dots that have a photoluminescent quantum yield (PLQY) approaching unity. This need makes the accurate, precise, and verifiable determination of the photoluminescence efficiency of QDs of utmost importance for the field. Here, we describe photothermal threshold quantum yield as a calorimetric method for measuring the PLQY of QDs in liquid dispersions. Taking the example of InP-based core/shell QDs, we detail the principles behind the analysis, and we benchmark results relative to the spectroscopic determination using an absolute PLQY measurement with an integrating sphere. We argue that the accuracy of the method for highly efficient emitters, and the simplicity of the data and the data analysis make photothermal threshold quantum yield well suited for certifying the photoluminescence efficiency of QDs.

Probing reversible photoluminescence alteration in CH3NH3PbBr3 colloidal quantum dots for luminescence-based gas sensing application

Colloidal CdSe/Zn(0.5)Cd(0.5)Se/ZnSe/ZnS core/multishell quantum dots (QDs) were synthesized by using the well developed successive ion layer adsorption and reaction (SILAR) technique. The UV-vis and PL spectra, TEM, X-ray diffraction and Raman measurement were performed to investigate the structure and optical properties of prepared QDs during the growth of shell layers, which indicated that the stress in CdSe core became stronger with the increasing shell thickness. Due to the gradual adjustment of the lattice parameters in the radial direction and the radial increase of the respective valence- and conduction-band offsets, the optical measurements show a significant enhancement in the photoluminescence quantum yield (QY) and an expedited radiative decay in QDs overcoated with thicker shell. The temperature-dependent optical spectra were measured, and the relation between the microstructure and the optical properties of these core/multishell quantum dots was discussed.

Red-emitting quantum dots (QDs) are particularly valuable in fluorescence-based chemical sensing, as longer-wavelength emission reduces background fluorescence and improves the signal-to-noise ratio in complex environments. CuInS2 (CIS) QDs are a promising cadmium-free alternative with tunable emission. However, red-shifting their photoluminescence (PL) peak often decreases their PL quantum yield (PLQY), which limits their further application. In this work, we synthesize CIS/ZnS core–shell QDs using the precursor thermal decomposition method. We further enlarged the CIS core to decrease its surface-to-volume ratio, thereby reducing interfacial defect density between the core and shell and narrowing the bandgap of the core, which achieves red PL emission with a peak wavelength of 668 nm and a PLQY of 82.3% in toluene. Subsequent ligand exchange with 3-mercaptopropionic acid (MPA) endows CIS/ZnS QDs with aqueous dispersibility. The resulting CIS/ZnS QDs with MPA ligands exhibit a high PLQY of 40.3% in a phosphate-buffered saline (PBS) aqueous solution at pH 8 and retain good storage stability for one month. Furthermore, the QDs enable selective detection of Cu2+ among 12 metal ions in PBS solutions, with clearly observable fluorescence quenching.