Search

Nanocrystalquantum dots (QDs) are attractive materials for applications as lasermedia because of their bright, size-tunable emission and the flexibilityafforded by colloidal synthesis. Nonradiative Auger recombination,however, hampers optical amplification in QDs by rapidly depletingthe population of gain-active multiexciton states. In order to elucidatethe role of Auger recombination in QD lasing and isolate its influencefrom other factors that might affect optical gain, we study two typesof CdSe/CdS core/shell QDs with the same core radii and the same totalsizes but different properties of the core/shell interface (“sharp”vs “smooth”). These samples exhibit distinctly differentbiexciton Auger lifetimes but are otherwise virtually identical. Thesuppression of Auger recombination in the sample with a smooth (alloyed)interface results in a notable improvement in the optical gain performancemanifested in the reduction of the threshold for amplified spontaneousemission and the ability to produce dual-color lasing involving boththe band-edge (1S) and the higher-energy (1P) electronic states. Wedevelop a model, which explicitly accounts for the multiexciton natureof optical gain in QDs, and use it to analyze the competition betweenstimulated emission from multiexcitons and their decay via Auger recombination.These studies re-emphasize the importance of Auger recombination controlfor the realization of real-life QD-based lasing technologies andoffer practical strategies for suppression of Auger recombinationvia “interface engineering” in core/shell structures. [ABSTRACT FROM AUTHOR]

We report scalable controlled synthesis of CdSe tetrapodswithhigh morphological uniformity based on the continuous precursor injection(CPI) approach with halide ligands. The CPI approach involves thesuccessive injection of precursors into the seed solution at a controlledrate so that the reaction condition remains in the kinetic growthregime. To initiate the successful development of tetrapod structure,the controlled amount of halide ligands are added during the reaction,which triggered the formation of wurtzite arms on {111}-facets ofthe zincblende seeds. The formation of the wurtzite phase is responsiblefor the halide-mediated displacement of oleate ligands, destabilizingthe embryonic CdSe phase on the zincblende seeds enabling the phasetransformation to more stable wurtzite phase on the hexagonal {111}-facets.On the basis of these halide-mediated phase transformation of CdSenanocrystals and the persistent kinetic growth induced by the CPIapproach, well-defined CdSe tetrapods with controlled arm length anddiameter have been produced in large quantity. [ABSTRACT FROM AUTHOR]

PbSe nanocrystal quantum dots (NQDs) are a promising active material for a range of optoelectronic devices, including solar cells, high-sensitivity infrared (IR) photodetectors, and IR-emitting diodes and lasers. However, device realization has been constrained by these NQDs' chemical instability toward oxidation, which leads to uncontrollable changes in optical and electronic properties. Here, we present a simple method to enhance the stability of PbSe NQDs against oxidation and to improve their optical properties through reaction with molecular chlorine. The chlorine molecules preferentially etch out surface Se ions and react with Pb ions to form a thin (1-2 monolayers) PbClx passivation layer which effectively prevents oxidation during long-term air exposure while passivating surface trap states to increase photoluminescence efficiency and decrease photocharging. Our method is simple, widely applicable to PbSe and PbS NQDs of a range of sizes, compatible with solution-based processes for fabricating NQD-based devices, and effective both in solution and in solid NQD films; thus, it is a practical protocol for facilitating advances over the full range of optoelectronic applications. [ABSTRACT FROM AUTHOR]

Utilizing the reactivity difference between TOPSe and TOPS, we synthesized InP@ZnSeS QDs with the composition gradient in a radial direction where ZnSe alleviated lattice strain and ZnS protected QDs from degradation so that we achieved QDs with high QE and photo/chemical stability. In terms of systematic investigation on the relationship between the shell nanostructure and QD stability, we demonstrated that QDs with thick gradient shells exhibited high QE and much enhanced stability against the shell degradation under UV irradiation, ligand exchange, or rigorous purification. This enhanced stability of InP@ZnSeS QDs is attributed to the improved uniformity of composition gradient shells, the efficient confinement of exciton wavefunctions, and the minimized surface oxidation and non-radiative decay via surface states generated by photo-oxidation or ligand exchange. Using InP@ZnSeS QDs with enhanced stability, we were able to demonstrate InP-based colloidal green-emitting QD-LEDs. Although the current status of InP@ZnSeS QDs is not fully optimized to realize practical optoelectronic devices, the approach taken in the present study (i.e., the composition gradient shell structure naturally made from reactivity difference in precursors) will give clues to facilitate the synthesis of InP QDs with advanced nanostructures. [ABSTRACT FROM AUTHOR]

A systematic analysis of the exciton-recombination zone within all-quantum dot (QD) multilayer films prepared by a layer-by-layer assembly method was made, using sensing QD layers in QD-based light-emitting diodes (QLEDs). Large area practical multicolored colloidal QLEDs were also demonstrated by patterning and placing variously colored QDs (red, orange, yellow-green, and green) in the exciton-recombination zone. [ABSTRACT FROM AUTHOR]

We demonstrated a facile synthesis of highly luminescent blue emitting Cd 1− xZn xS/ZnS core/shell structured nanocrystals (NCs) in straightforward and reproducible manner. The alloyed Cd 1− xZn xS cores with homogeneity in both size and composition were prepared by introducing S precursors (S dissolved in the noncoordinating solvent (1-octadecene)) into the mixed solution of Cd−Oleate (Cd(OA) 2) and Zn−Oleate (Zn(OA) 2) at elevated temperature (300 °C). ZnS shells were successively overcoated on the prepared cores by the second injection of S precursors (S powder dissolved in tributylphosphine, TBPS) directly into the reactor with existing alloyed Cd 1− xZn xS NC cores without any purification steps. The prepared NCs exhibit strong band edge emission with high photoluminescent quantum yield (PL QY, up to 80%) and narrow spectral bandwidth (fwhm < 25 nm), which is believed to originate from the successful growth of ZnS shell layers on the Cd 1− xZn xS cores and the interfacial compatibility between Cd 1− xZn xS cores and the ZnS shell layers through the intradiffusion of Zn atoms from the ZnS shells into the Cd 1− xZn xS cores during the shell formation reaction. The emission wavelength (PL λ max.) of Cd 1− xZn xS/ZnS core/shell NCs was finely tuned from violet (415 nm) to blue (461 nm) by adjusting the amount of S precursors in the first injection (S in 1-octadecene) and thus changing actual Cd content ratio in the alloyed Cd 1− xZn xS cores (0.49 ≤ x≤ 0.76). Furthermore, multigram (3 g) scale production of Cd 1− xZn xS/ZnS core/shell NCs with narrow size distribution and spectral bandwidth was also demonstrated. [ABSTRACT FROM AUTHOR]

We demonstrate a single-step synthetic method for highly luminescent (i.e., quantum yield up to 80%) and stable quantum dots (QDs) by using the reactivity difference between Cd and Zn precursors and that between Se and S precursors. A wide range of emission wavelengths (500−610 nm) with a narrow fwhm (<35 nm) is obtained by changing the ratios of the precursors. Under the reaction conditions selected, Cd- and Se (with a bit of S)-based cores are formed first and Zn- and S-based shells are formed successively; therefore, the QDs have a core/shell structure with composition gradients, which relieve the lattice mismatch between core and shells. The QDs are characterized using the combined techniques of HR-TEM, UV–vis, PL spectroscopy, and ICP-AES. The QDs also have energy gradients depending on their compositions in a radial direction, which energetically confine carriers (electrons and holes) to the cores. This leads to the stability of QDs during their surface passivation from oleic acid to mercaptopropionic acid and ensures their processibility for further purposes such as optoelectronic and biological applications. [ABSTRACT FROM AUTHOR]

Gallium ion incorporation into silver indium gallium sulfide nanocrystals is investigated by various methods, including photoluminescence (PL) and X-ray photoelectron spectroscopy. The ZnS shell-growth enhances a PL quantum yield of up to 16%, with which the quantum dot light-emitting diode was successfully fabricated. [ABSTRACT FROM AUTHOR]

We demonstrate deep blue light-emitting diodes based on chemically synthesized Cd1[?]xZnxS@ZnS quantum dots (QDs). Composite films of poly-(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD) and 4,4',N,N'-diphenylcarbazole (CBP) are employed for facilitated hole injection into Cd1[?]xZnxS@ZnS QDs and uniform QD deposition. The fabricated devices possess moderate turn-on voltage (6 V) and external quantum efficiency (0.1-0.3%), and exhibit color-saturated blue emission with a narrow spectral bandwidth of full width at half maximum <25 nm (Commission Internationale de l'Eclairage (CIE) coordinates of (0.169, 0.024) and (0.156, 0.028) for devices with electroluminescence (EL) lmax at 434 and 454 nm, respectively). Most of the emission originates from the Cd1[?]xZnxS@ZnS QD layers (99% of the total EL emission). [ABSTRACT FROM AUTHOR]

Vortex formation is observed in transmission electron microscopy (TEM) images of colloidal CdS nanorod (NR) aggregates. Upon adding polar (poor) solvents into the NR dispersion, the NRs passivated with alkylphosphonic acids start flocculating. When NR clusters are deposited on a TEM grid, the tips of the NRs are immobilized on the substrate, and capillary forces give rise to a net inward tilting as the solvent evaporates, eventually leading to the formation of vortices. Solvent polarity turns out to play a critical role in controlling the structure of vortices. Fluorescence lifetime imaging microscopy (FLIM) of the ensembles reveals that the vortex structures have profound influence on the energy transfer rate within the NR assembly. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for photovoltaic devices. An important LSC characteristic is a concentration factor (C), which is defined as the ratio of the output and the input photon flux densities. This parameter can be also thought of as an effective enlargement factor of a solar cell active area. On the basis of thermodynamic considerations, the C-factor can reach extremely high values that exceed those accessible with traditional concentrating optics. In reality, however, the best reported values of C are around 30. Here we demonstrate that using a new type of high-emissivity quantum dots (QDs) incorporated into a specially designed cavity, we are able to achieve the C of ~62 for spectrally integrated emission and ~120 for the red portion of the photoluminescence spectrum. The key feature of these QDs is a seed/quantum-well/thick-shell design, which allows for obtaining a high emission quantum yield (>95%) simultaneously with a large LSC quality factor (QLSC of ~100) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. By incorporating the QDs into a specially designed cavity equipped with a top selective reflector (a Bragg mirror or a thin silver film), we are able to effectively recycle reemitted light achieving light trapping coefficients of ~85%. The observed performance of these devices is in remarkable agreement with analytical modeling, which allows us to project that the applied approach should allow one to boost the spectrally integrated concentration factors to more than 100 by further improving light trapping and/or increasing QLSC. [ABSTRACT FROM AUTHOR]

Growth of monodisperse indium phosphide (InP) quantum dots (QDs) represents a pressing demand in display applications, as size uniformity is related to color purity in display products. Here, we report the colloidal synthesis of InP QDs in the presence of Zn precursors in which size uniformity is markedly enhanced as compared to the case of InP QDs synthesized without Zn precursors. Nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and mass spectrometry analyses on aliquots taken during the synthesis allow us to monitor the appearance of metal-phosphorus complex intermediates in the growth of InP QDs. In the presence of zinc carboxylate, intermediate species containing Zn-P bonding appears. The Zn-P intermediate complex with P(SiMe3)3 exhibits lower reactivity than that of the In-P complex, which is corroborated by our prediction based on density functional theory and electrostatic potential charge analysis. The formation of a stable Zn-P intermediate complex results in lower reactivity, which enables slow growth of QDs and lowers the extreme reactivity of P(SiMe3)3, hence monodisperse QDs. Insights from experimental and theoretical studies advance mechanistic understanding and control of nucleation and growth of InP QDs, which are key to the preparation of monodisperse InP-based QDs in meeting the demand of the display market. [ABSTRACT FROM AUTHOR]

Herein, we report unique features of the assemblies of tetrapod-shaped colloidal nanocrystals (TpNCs) with lengthy arms applicable to flexible thin-film transistors. Due to the extended nature of tetrapod geometry, films made of the TpNC assemblies require reduced numbers of inter-NC hopping for the transport of charge carriers along a given channel length; thus, enhanced conductivity can be achieved compared to those made of typical spherical NCs without arms. Moreover, electrical conduction through the assemblies is tolerant against mechanical bending because interconnections between TpNCs can be well-preserved under bending. Interestingly, both the conductivity of the assemblies and their mechanical tolerance against bending are improved with an increase in the length of tetrapod arms. The arm length-dependency was demonstrated in a series of CdSe TpNC assemblies with different arm lengths (l = 0-90 nm), whose electrical conduction was modulated through electrolyte gating. From the TpNCs with the longest arm length included in the study (l = 90 nm), the film conductivity as high as 20 S/cm was attained at 3 V of gate voltage, corresponding to electron mobility of >10 cm²/(V s) even when evaluated conservatively. The high channel conductivity was retained (~90% of the value obtained from the flat geometry) even under high bending (bending radius = 5 mm). The results of the present study provide new insights and guidelines for the use of colloidal nanocrystals in solution-processed flexible electronic device applications. [ABSTRACT FROM AUTHOR]

Ratiometric pressure sensitive paints (r-PSPs) are all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist of a porous binder embedding mixtures of a reference and a sensor chromophore exhibiting oxygen-insensitive and oxygen-responsive luminescence, respectively. Here, we demonstrate the first example of an r-PSP based on a single two-color emitter that removes limitations of r-PSPs based on chromophore mixtures such as different temperature dependencies of the two chromophores, cross-readout between the reference and sensor signals and phase segregation. In our approach, we utilize a novel "double-sensor" r-PSP that features two spectrally separated emission bands with opposite responses to the O2 pressure, which boosts the sensitivity with respect to traditional reference-sensor pairs. Specifically, we use two-color-emitting dot-in-bulk CdSe/CdS core/shell nanocrystals, exhibiting red and green emission bands from their core and shell states, whose intensities are respectively enhanced and quenched in response to the increased oxygen partial pressure that effectively tunes the position of the nanocrystal's Fermi energy. This leads to a strong and reversible ratiometric response at the single particle level and an over 100% enhancement in the pressure sensitivity. Our proof-of-concept r-PSPs further exhibit suppressed cross-readout thanks to zero spectral overlap between the core and shell luminescence bands and a temperature-independent ratiometric response between 0 and 70 °C. [ABSTRACT FROM AUTHOR]

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminescent solar concentrators; and control of intraband relaxation for enhanced carrier multiplication in advanced QD photovoltaics. We examine the considerable recent progress on these multiple fronts of nanocrystal research, which has resulted in the first commercialized QD technologies. These successes explain the continuing appeal of this field to a broad community of scientists and engineers, which in turn ensures even more exciting results to come from future exploration of this fascinating class of materials. [ABSTRACT FROM AUTHOR]

We describe the synthesis of heterotelechelic polysarcosine polymers and their use as multidentate ligands in the preparation of stable water-soluble quantum dots (QDs). Orthogonally functionalized polysarcosine with amine and dibenzocyclooctyl (DBCO) end groups is obtained by ring-opening polymerization of N-methylglycine N-carboxyanhydride with DBCO amine as initiator. In a first postpolymerization modification step, the future biological activity of the polymeric ligands is adjusted by modification of the amine terminus. Then, in a second postpolymerization modification step, azide functionalized di- and tridentate anchor compounds are introduced to the DBCO terminus of the polysarcosine via strain-promoted azide-alkyne cycloaddition (SPAAC). Through the separate synthesis of the anchor compounds, it is possible to ensure reproducible introduction of a well-defined number of multiple anchor groups to all polymers studied. Finally, the obtained multidentate polymeric ligands are successfully used in the ligand exchange procedures to yield stable, water-soluble QDs. As polysarcosine-based ligands can provide biocompatibility, prevent nonspecific interactions, and simultaneously enable specific targeting, the systems presented here are promising candidates to provide QDs well suitable for ex vivo analytics or bioimaging. [ABSTRACT FROM AUTHOR]

We examine the effects of chlorine-passivation of Cd surface atoms on photocatalytic H2O reduction by CdSe NCs. Transient absorption spectroscopy reveals that Cl passivation removes electron trap states in CdSe NCs, which is also reflected in an increase of photoluminescence quantum yield, e.g., from 9 to 22% after the Cl treatment. Size-tunable energy states in CdSe NCs enable the systematic investigation of surface defects and their effect on the photocatalytic hydrogen generation rate. It turns out that, depending on band-edge energy levels, the surface trap states may enhance or inhibit photocatalysis. Cl-treated CdSe NCs larger than 2.7 nm show a higher hydrogen evolution rate than untreated CdSe NCs of the same size as Cl treatment removes trap states with energy below the H2O reduction potential. In contrast, the same Cl treatment does not increase the photocatalytic rate of CdSe NCs smaller than 2.7 nm because both the conduction band edge and trap states are above the water reduction potential. The size-dependence of the effect of Cl treatment suggests that electron trap states in CdSe may promote photocatalytic activity by enhancing charge separation. [ABSTRACT FROM AUTHOR]

We report synthesis of PbSe nanorods (NRs) and PbSe/CdSe axial heterojunction NRs via direct Cd-to-Pb cation exchange in CdSe NRs. Use of suited ligandâ€"cation combinations enables the cation exchange while keeping the nanomaterial morphology intact. For example, solvation of Cd2+ using oleylamine (OLA) allows for the cation exchange process, which would not be possible by using oleic acid instead of OLA. A mild cation exchange process, such as mixing Pb-oleate and OLA with CdSe NRs at 130 or 150 °C, results in anisotropic replacement of CdSe into PbSe along the â0001â© direction of wurtzite CdSe, and a partial conversion leads to the formation of heterostructure NRs containing axial CdSe/PbSe heterojunctions. While the cation exchange proceeds at both tips of CdSe NRs, exchange appears to be faster on (0001Ì...) planes. Binding energy calculation based on density functional theory reveals that OLA binds strongly to the (0001Ì...) facet of CdSe NRs, leading to asymmetric cation exchange. This protocol to convert CdSe nanocrystals directly into PbSe broadens the design range of CdSe/PbSe heterojunction nanomaterials potentially with various morphologies because template CdSe nanocrystals can be prepared in different shapes via colloidal synthesis. [ABSTRACT FROM AUTHOR]

We present facile synthesis of bright CdS/CdSe/CdS@SiO2 nanoparticles with 72% of quantum yields (QYs) retaining ca 80% of the original QYs. The main innovative point is the utilization of the highly luminescent CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) as silica coating seeds. The significance of inorganic semiconductor shell passivation and structure design of quantum dots (QDs) for obtaining bright QD@SiO2 is demonstrated by applying silica encapsulation via reverse microemulsion method to three kinds of QDs with different structure: CdSe core and 2 nm CdS shell (CdSe/CdS-thin); CdSe core and 6 nm CdS shell (CdSe/CdS-thick); and CdS core, CdSe intermediate shell and 5 nm CdS outer shell (CdS/CdSe/CdS-SQW). Silica encapsulation inevitably results in lower photoluminescence quantum yield (PL QY) than pristine QDs due to formation of surface defects. However, the retaining ratio of pristine QY is different in the three silica coated samples; for example, CdSe/CdS-thin/SiO2 shows the lowest retaining ratio (36%) while the retaining ratio of pristine PL QY in CdSe/CdS-thick/SiO2 and SQW/SiO2 is over 80% and SQW/SiO2 shows the highest resulting PL QY. Thick outermost CdS shell isolates the excitons from the defects at surface, making PL QY relatively insensitive to silica encapsulation. The bright SiO2-coated SQW sample shows robustness against harsh conditions, such as acid etching and thermal annealing. The high luminescence and long-term stability highlights the potential of using the SQW/SiO2 nanoparticles in bio-labeling or display applications. [ABSTRACT FROM AUTHOR]