n Song Li

High-Efficiency, Low Turn-on Voltage Blue-Violet Quantum-Dot-Based Light-Emitting Diodes

We report high-efficiency blue-violet quantum-dot-based light-emitting diodes (QD-LEDs) by using high quantum yield ZnCdS/ZnS graded core-shell QDs with proper surface ligands. Replacing the oleic acid ligands on the as-synthesized QDs with shorter 1-octanethiol ligands is found to cause a 2-fold increase in the electron mobility within the QD film. Such a ligand exchange also results in an even greater increase in hole injection into the QD layer, thus improving the overall charge balance in the LEDs and yielding a 70% increase in quantum efficiency. Using 1-octanethiol capped QDs, we have obtained a maximum luminance (L) of 7600 cd/m(2) and a maximum external quantum efficiency (ηEQE) of (10.3 ± 0.9)% (with the highest at 12.2%) for QD-LEDs devices with an electroluminescence peak at 443 nm. Similar quantum efficiencies are also obtained for other blue/violet QD-LEDs with peak emission at 455 and 433 nm. To the best of our knowledge, this is the first report of blue QD-LEDs with ηEQE > 10%. Combined with the low turn-on voltage of ∼2.6 V, these blue-violet ZnCdS/ZnS QD-LEDs show great promise for use in next-generation full-color displays.

Synthesis and assembly of monodisperse spherical Cu2S nanocrystals.

High-quality monodisperse Cu(2)S nanocrystals (sizes from 2 nm to 20 nm) have been successfully synthesized by the reaction of copper stearate (CuSt(2)) and dodecanethiol (DDT) in 1-octadecene (ODE). The nanocrystals were characterized using X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), and transmission electron microscopy (TEM). These as-prepared Cu(2)S nanocrystals with certain sizes have been found with good self-assembly behaviors, and they were easily to assemble into two-dimensional and three-dimensional superlattice structures. DDT served as both sulfur source and capping ligand, and was found a key factor to affect the growth and the self-assembly behaviors of the Cu(2)S nanocrystals.

Columnar Self-Assembly of Cu2S Hexagonal Nanoplates Induced by Tin(IV)−X Complex as Inorganic Surface Ligand

We have prepared columnar self-assembled Cu(2)S hexagonal nanoplates induced by a Sn-X complex for the first time and demonstrated that the Sn-X complex can affect not only the morphology of the nanocrystals but also the self-assembly ability of the nanocrystals.

High quality synthesis of monodisperse zinc-blende CdSe and CdSe/ZnS nanocrystals with a phosphine-free method

Highly monodisperse zinc blende CdSe nanocrystals have been synthesized by using different phosphine-free Se precursors successfully. To understand the reaction mechanism and obtain high quality CdSe nanocrystals, the effects on the use of different Se and/or Cd precursors, the adjustment of the molar ratios between Cd and Se precursors, and the selection of suitable reaction and growth temperatures have been studied in details. Absorption spectrum, fluorescence spectrum, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used for the characterization of synthesized CdSe nanocrystals. The quality of as-prepared CdSe nanocrystals was reached the same high level compared with the method using phosphine selenium precursors, its quantum yields were among 30 to 60% and photoluminescence (PL) full width at half-maximum (FWHM) was well controlled between 22 and 28 nm. As core, such zinc blende CdSe nanocrystals were also used to synthesize CdSe/ZnS, CdSe/CdS, and CdSe/CdS/ZnS core-shell nanocrystals. The quantum yields of as-prepared core-shell nanocrystals were among 50 to 80%. Large-scale syntheses of such core-shell nanocrystals have been successfully demonstrated and as many as 3 g of high quality CdSe/ZnS nanocrystals were easily synthesized with the use of only low-cost, green, and environmentally friendlier reagents.

Inorganic Sn–X complex ligands capped CuInS2 nanocrystals with high electron mobility

We report a facile method for the synthesis of size-controlled triangular CuInS2 (CIS) semiconductor nanocrystals (NCs) in the organic phase, and then, molecular metal chalcogenide complexes capped CIS NCs can be synthesized by exchanging original organic compounds with (NH4)4Sn2S6 inorganic ligands in environmentally benign solvent. The properties of CIS NCs (coated by both organic and inorganic ligands) were characterized by UV–Vis spectroscopy, fourier transform infrared, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, and dynamic light scattering. CuInS2 NCs (before and after ligand exchange) films were spin coated on cleaned ITO glass substrates, and the charge transport properties were detected by current-voltage characteristic. We observed that the ligands on the surface of CIS NCs have been exchanged successfully, and the electrical transparency of (NH4)4Sn2S6-CIS NCs films was obviously increased than CIS NCs with organic capping ligands.

Highly Efficient Blue–Green Quantum Dot Light-Emitting Diodes Using Stable Low-Cadmium Quaternary-Alloy ZnCdSSe/ZnS Core/Shell Nanocrystals

High-quality blue-green emitting ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell quantum dots (QDs) have been synthesized by a phosphine-free method. The quantum yields of as-synthesized ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell QDs can reach 50-75% with emissions between 450 and 550 nm. The emissions of such core/shell QDs are not susceptible to ligand loss through the photostability test. Blue-green light-emitting diodes (LEDs) based on the low-cadmium ZnxCd(1-x)S(1-y)Se(y)/ZnS core/shell QDs have been successfully demonstrated. Composite films of poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) and ZnO nanoparticle layers were chosen as the hole-transporting and the electron-transporting layers, respectively. Highly bright blue-green QD-based light-emitting devices (QD-LEDs) showing maximum luminance up to 10000 cd/m(2), in particular, the blue QD-LEDs show an unprecedentedly high brightness over 4700 cd/m(2) and peak external quantum efficiency (EQE) of 0.8%, which is the highest value ever reported. These results signify a remarkable progress in QD-LEDs and offer a practicable platform for the realization of QD-based blue-green display and lighting.

Efficient and Bright Colloidal Quantum Dot Light-Emitting Diodes via Controlling the Shell Thickness of Quantum Dots

In this paper, we use a simple device architecture based on solution-processed ZnO nanoparticles (NPs) as the electron injection/transport layer and bilayer structure of poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) as the hole injection/transport layer to assess the effect of shell thickness on the properties of quantum-dot-based light emitting diodes (QD-LEDs), comprising CdSe/CdS/ZnS core-shell QDs as the emitting layer. QDs with varying shell thickness were assessed to determine the best option of shell thickness, and the best improvement in device performance was observed when the shell thickness was 2.1 nm. Thereafter, different emissions of QDs, but with optimized same shell thickness (∼2.1 nm), were selected as emitters to be fabricated into same structured QD-LEDs. Highly bright orange-red and green QD-LEDs with peak luminances up to ∼30 000 and ∼52 000 cd m(-2), and power efficiencies of 16 and 19.7 lm W(-1), respectively, were demonstrated successfully. These results may demonstrate a striking basic prototype for the commercialization of QD-based displays and solid-state lightings.

Investigation on type-II Cu2S–CdS core/shell nanocrystals: synthesis and characterization

Type-II Cu2S–CdS core/shell (including Cu2S/CdS and CdS/Cu2S) semiconductor nanocrystals (NCs), which can produce efficient spatial separation of electrons and holes between the core and shell, were successfully synthesized. The results from absorption spectra, photoluminescence (PL) spectra, X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and PL lifetime measurements all confirmed the formation of the type-II core/shell NCs. The emission wavelength of the type-II Cu2S/CdS core/shell NCs can be readily tuned between 515 nm and 760 nm through two methods: core size control and shell thickness control. The highest PL quantum yield that the Cu2S/CdS core/shell NCs reached was ∼12% by controlling the shell thickness, while the core Cu2S nanocrystals had no PL at all. The PL decay data revealed that Cu2S/CdS NCs had a dual exponential characteristic and the PL emission with the longest lifetime (more than 1400 ns) occupied more than 85%. After overcoating ZnS, a Cu2S/CdS/ZnS core/shell1/shell2 structure was formed, which resulted in a blue-shift of the emission wavelength. Using the synthesis strategy of Cu2S/CdS, CdS/Cu2S type-II NCs also produced tunable emissions in the visible range.

Phosphine-free synthesis of Zn1−xCdxSe/ZnSe/ZnSexS1−x/ZnS core/multishell structures with bright and stable blue–green photoluminescence

Highly photoluminescent (PL) Zn1−xCdxSe/ZnSe/ZnSexS1−x/ZnS core/multishell nanocrystals were successfully synthesized by a phosphine-free method. Alloyed ZnSexS1−x was chosen as the shell material to epitaxially grow on pre-synthesized Zn1−xCdxSe nanocrystals. By decreasing the molar ratio between Se and S in ZnSexS1−x, the shell composition gradually changed from ZnSe to ZnS. The PL emissions of these core/multishell nanocrystals can be tuned from 450 to 530 nm. The PL quantum yields (QYs) of as-synthesized nanocrystals reached 50 to 75% with full width at half maximum (FWHM) between 28 and 45 nm. By an organic–aqueous phase transfer method, such core/multishell nanocrystals can be transferred into water successfully using an amphiphilic oligomer (polymaleic acid n-hexadecanol ester) as a surface coating agent. It was found that the shell thicknesses of the as-synthesized core/multishell nanocrystals dramatically affected the phase transfer efficiency. Stability tests showed that the water-soluble Zn1−xCdxSe/ZnSe/ZnSexS1−x/ZnS core/multishell nanocrystals with 10 monolayers of shells had very high PL QY and stability in various physiological conditions.

Targeting of embryonic stem cells by peptide-conjugated quantum dots.

Background Targeting stem cells holds great potential for studying the embryonic stem cell and development of stem cell-based regenerative medicine. Previous studies demonstrated that nanoparticles can serve as a robust platform for gene delivery, non-invasive cell imaging, and manipulation of stem cell differentiation. However specific targeting of embryonic stem cells by peptide-linked nanoparticles has not been reported. Methodology/Principal Findings Here, we developed a method for screening peptides that specifically recognize rhesus macaque embryonic stem cells by phage display and used the peptides to facilitate quantum dot targeting of embryonic stem cells. Through a phage display screen, we found phages that displayed an APWHLSSQYSRT peptide showed high affinity and specificity to undifferentiated primate embryonic stem cells in an enzyme-linked immunoabsorbent assay. These results were subsequently confirmed by immunofluoresence microscopy. Additionally, this binding could be completed by the chemically synthesized APWHLSSQYSRT peptide, indicating that the binding capability was specific and conferred by the peptide sequence. Through the ligation of the peptide to CdSe-ZnS core-shell nanocrystals, we were able to, for the first time, target embryonic stem cells through peptide-conjugated quantum dots. Conclusions/Significance These data demonstrate that our established method of screening for embryonic stem cell specific binding peptides by phage display is feasible. Moreover, the peptide-conjugated quantum dots may be applicable for embryonic stem cell study and utilization.