Pump-intensity- and shell-thickness-dependent evolution of photoluminescence blinking in individual core/shell CdSe/CdS nanocrystals

We report a systematic study of photoluminescence (PL) intensity and lifetime fluctuations in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. We show that while at low pump intensities PL blinking in thin-shell (4-7 monolayers, MLs) NQDs can be described by random switching between two states of high (ON) and low (OFF) emissivities, it changes to the regime with a continuous distribution of ON intensity levels at high pump powers. A similar behavior is observed in samples with a medium shell thickness (10-12 MLs) without, however, the PL intensity ever switching to a complete "OFF" state and maintaining ca. 30% emissivity ("gray" state). Further, our data indicate that highly stable, blinking-free PL of thick-shell (15-19 MLs) NQDs ("giant" or g-NQDs) is characterized by nearly perfect Poisson statistics, corresponding to a narrow, shot-noise limited PL intensity distribution. Interestingly, in this case the PL lifetime shortens with increasing pump power and the PL decay may deviate from monoexponential. However, the PL intensity distribution remains shot-noise limited, indicating the absence of significant quantum yield fluctuations at a given pump power intensity during the experimental time window.     

Amphoteric CdSe nanocrystalline quantum dots

The nanocrystal quantum dot (NQD) charge states strongly influence their electrical transport properties in photovoltaic and electroluminescent devices, optical gains in NQD lasers, and the stability of the dots in thin films. We report a unique electrostatic nature of CdSe NQDs, studied by electrophoretic methods. When we submerged a pair of metal electrodes, in a parallel plate capacitor configuration, into a dilute solution of CdSe NQDs in hexane, and applied a DC voltage across the pair, thin films of CdSe NQDs were deposited on both the positive and the negative electrodes. Extensive characterizations including scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) and Raman studies revealed that the films on both the positive and the negative electrodes were identical in every respect, clearly indicating that: (1)?a fraction (<1%) of the CdSe NQDs in free form in hexane solution are charged and, more importantly, (2)?there are equal numbers of positive and negative CdSe NQDs in the hexane solution. Experiments also show that the number of deposited dots is at least an order of magnitude higher than the number of initially charged dots, indicating regeneration. We used simple thermodynamics to explain such amphoteric nature and the charging/regeneration of the CdSe NQDs.     

Competition between Auger Recombination and Hot‐Carrier Trapping in PL Intensity Fluctuations of Type II Nanocrystals

Performing time‐tagged, time‐correlated, single‐photon‐counting studies on individual colloidal nanocrystal quantum dots (NQDs), the evolution of photoluminescence (PL) intensity‐fluctuation behaviors in near‐infrared (NIR) emitting type II, InP/CdS core‐shell NQDs is investigated as a function of shell thickness. It is observed that Auger recombination and hot‐carrier trapping compete in defining the PL intensity‐fluctuation behavior for NQDs with thin shells, whereas the role of hot‐carrier trapping dominates for NQDs with thick shells. These studies further reveal the distinct ramifications of altering either the excitation fluence or repetition rate. Specifically, an increase in laser pump fluence results in the creation of additional hot‐carrier traps. Alternately, higher repetition rates cause a saturation in hot‐carrier traps, thus activating Auger‐related PL fluctuations. Furthermore, it is shown that Auger recombination of negatively charged excitons is suppressed more strongly than that of positively charged excitons because of the asymmetry in the electron‐hole confinement in type II NQDs. Thus, this study provides new understanding of how both NQD structure (shell thickness and carrier‐separation characteristics) and excitation conditions can be used to tune the PL stability, with important implications for room‐temperature single‐photon generation. Specifically, the first non‐blinking NQD capable of single‐photon emission in the near‐infrared spectral regime is described.     

'Giant' CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance

We use a simple device architecture based on a poly(3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated indium tin oxide anode and a LiF/Al cathode to assess the effects of shell thickness on the properties of light-emitting diodes (LEDs) comprising CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as the emitting layer. Specifically, we are interested in determining whether LEDs based on thick-shell nanocrystals, so-called "giant" NQDs, afford enhanced performance compared to their counterparts incorporating thin-shell systems. We observe significant improvements in device performance as a function of increasing shell thickness. While the turn-on voltage remains approximately constant for all shell thicknesses (from 4 to 16 CdS monolayers), external quantum efficiency and maximum luminance are found to be about one order of magnitude higher for thicker shell nanocrystals (≥13 CdS monolayers) compared to thinner shell structures (<9 CdS monolayers). The thickest-shell nanocrystals (16 monolayers of CdS) afforded an external quantum efficiency and luminance of 0.17% and 2000 Cd/m(2), respectively, with a remarkably low turn-on voltage of ~3.0 V.     

Photocurrent imaging of nanocrystal quantum dots on single-walled carbon nanotube device

Semiconductor nanocrystal quantum dots (NQDs) are considered an attractive candidate for use in optoelectronic applications due to the ease of band gap control provided by varying the particle size. To increase the efficiency of NQDs when practically applied in devices, researchers have introduced the concept of coupling of NQDs to one-dimensional nanostructures such as single-walled carbon nanotubes (SWCNTs), which have a ballistic conducting channel. In the present study, NQDs of CdSe core and CdSe/ZnS are used as light absorbing building blocks. SWCNTs and functionalized NQDs are non-covalently coupled using pyridine molecules in order to maintain their electronic structures. To measure the electrical signals from the device, a NQDs-SWCNT hybrid nanostructure is fabricated as a field-effect transistor (FET) using the dielectrophoresis (DEP) method. A confocal scanning microscope was used to scan the devices using a diffraction-limited laser spot and the photocurrent was recorded as a function of the position of the laser spot. To improve the performance of detecting small electronic signal with high signal-to-noise ratio we used a lock-in technique with an intensity-modulated laser. In this paper, we have demonstrated that detection of local photoconductivity provides an efficient means to resolve electronic structure modulations along NQDs-SWCNT hybrid nanostructures.     

CdSe/Cd1−x Zn x S core/shell quantum dots with tunable emission: growth and morphology evolution

We demonstrate an organic synthesis to fabricate hydrophobic core/shell CdSe/Cd_1?x Zn _x S quantum dots (QDs) with tunable photoluminescence (PL) between green and red at relatively low temperature using trioctylphosphine S reacted directly with cadmium and zinc acetate. A seeded growth strategy was used for preparing large CdSe cores. Large CdSe cores revealed a rod-like morphology while small one exhibited a spherical shape. Being coated with a Cd_1?x Zn _x S shell on spherical CdSe cores with an average size of 3.9 nm in diameter, core/shell QDs exhibited a cubic morphology (a length of 5 nm). In contrast, the core/shell QDs created using a small core (3.3 nm in diameter) show a spherical morphology. Namely, the anisotropic aggregation behavior of CdS monomers on CdSe cores occurs when the rod-like core is coated with a Cd_1?x Zn _x S shell. CdS interlayer plays an important role for such morphology evolution because all CdSe cores with a pure ZnS shell exhibited a spherical morphology. The PL properties of CdSe/Cd_1?x Zn _x S core/shell QDs depended strongly on the size and morphology of the cores. The QDs revealed a narrow and tunable PL spectrum. It is believed that this facile strategy can be extended to synthesize other core–shell QDs at low temperature.     

High color rendering index white LED based on nano-YAG:Ce3+ phosphor hybrid with CdSe/CdS/ZnS core/shell/shell quantum dots

To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce³⁺ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5?lm/W, (0.3264, 0.3255), 91 and 4580?K, respectively.     

水溶性量子点制备条件的优化及表面修饰

以L-半胱氨酸为稳定剂在水溶液中合成CdSe纳米粒子,研究了水浴时间、水浴温度、不同L-半胱氨酸/Cd/Se比例、pH值等因素对其荧光光谱的影响,确定了最佳的合成方案.用CdS对其表面进行修饰,采用透射电镜、X射线衍射、光谱法等表征了Cdse/CdS核壳结构颗粒的形成,结果表明该纳米粒子发光强度明显高于单一的CdSe量子点,光谱峰位置有所红移;合成条件会显著影响CASe/CAS核壳结构量子点的荧光性能.     

Cadmium sulfide and cadmium selenide/cadmium sulfide nanoparticles stabilized in water with poly(cysteine acrylamide)

Cysteine acrylamide (N-acryloyl L-cysteine) stabilizes CdS nanoparticles as the particles form in aqueous dispersions. Cysteine acrylamide also exchanges for citrate on the surfaces of CdSe and core/shell CdSe/CdS nanoparticles to provide greater stability. Heating of the nanoparticle dispersions polymerizes the cysteine acrylamide on the surface to form a more efficient polydentate stabilizer. The polymer-coated nanoparticle dispersions are colloidally stable even after removal of low molecular weight solutes by dialysis. Emission quantum yields of the polymer-coated CdSe and CdSe/CdS samples were 0.9% and 2.6%, respectively, after aging of the samples in light. CdSe/CdS coated with poly(cysteine acrylamide) is colloidally stable for at least two years in the dark at 5 degrees C.     

巯基包覆CdSe和CdSe/CdS核壳纳米晶的水相合成与表征

利用水相合成的方法制备了巯基包覆的具有较高荧光量子产率的CdSe和CdSe／CdS纳米晶．水相合成方法的优点是原料低廉、安全可靠和重复性高，缺点是纳米晶的尺寸分布较宽，发光效率不是很高．采用X-射线粉末衍射、吸收和荧光等光谱手段对纳米晶的平均尺度、粒径分布、晶体结构及发光特性进行了表征。在77K到300K的温度范围内，随着温度降低，CdSe纳米晶的发光峰逐渐蓝移，而CdSe／Cds纳米晶发光峰位基本不随温度变化而变化．此外，在325nm激光辐照下，CdSe／CdS纳米晶的荧光寿命比CdSe纳米晶延长了6倍左右，稳定性大幅度提高．以上结果表明，核壳结构的CdSe／CdS纳米晶具有较高的发光效率和良好的稳定性，具有广阔的应用前景．     

Cover Picture: Bright White‐Light Emission from Semiconductor Nanocrystals: by Chance and by Design (Adv. Mater. 4/2007)

Onionlike core/shell/shell (CdSe/ZnS/CdSe) nanocrystals are designed to emit in the yellow–orange regime from the CdSe core and in the blue regime from the CdSe shell. By changing the synthesis parameters, the intensity of emission from the shell can be varied, thereby tuning the shade of the emitted light, as reported Sameer Sapra and co‐workers on p. 569.     

High‐Gain 200 ns Photodetectors from Self‐Aligned CdS–CdSe Core–Shell Nanowalls

1D core–shell heterojunction nanostructures have great potential for high‐performance, compact optoelectronic devices owing to their high interface area to volume ratio, yet their bottom‐up assembly toward scalable fabrication remains a challenge. Here the site‐controlled growth of aligned CdS–CdSe core–shell nanowalls is reported by a combination of surface‐guided vapor–liquid–solid horizontal growth and selective‐area vapor–solid epitaxial growth, and their integration into photodetectors at wafer‐scale without postgrowth transfer, alignment, or selective shell‐etching steps. The photocurrent response of these nanowalls is reduced to 200 ns with a gain of up to 3.8 × 10³ and a photoresponsivity of 1.2 × 10³ A W^?1, the fastest response at such a high gain ever reported for photodetectors based on compound semiconductor nanostructures. The simultaneous achievement of sub‐microsecond response and high‐gain photocurrent is attributed to the virtues of both the epitaxial CdS–CdSe heterojunction and the enhanced charge‐separation efficiency of the core–shell nanowall geometry. Surface‐guided nanostructures are promising templates for wafer‐scale fabrication of self‐aligned core–shell nanostructures toward scalable fabrication of high‐performance compact photodetectors from the bottom‐up.     

Infrared photodiode based on colloidal PbSe nanocrystal quantum dots

We report in this paper our studies on the photoconductivity and photovoltaic effects of colloidal PbSe nanocrystal quantum dots (NQDs) which were embedded in conductive polymer matrices to form hybrid polymer/NQD infrared photodiodes. The generation of photocarriers in PbSe NQDs and their transport in NQD-polymer composites were described by a simplified band diagram picture of the device. Both photocurrent and photovoltage outputs were measured from the NQD-incorporated photodiode upon the illumination of near-infrared (NIR) light, whereas the net polymer-based devices do not exhibit any photoresponsivity. The intensity dependence of the photocurrent indicates the pseudomonomolecular recombination kinetics in the NQD-polymer composite. The measured photocurrent spectrum is consistent with the absorption characteristic of PbSe NQDs. Further enhancement of the photodiode efficiency can be achieved by engineering the nanocrystal surface to reduce the potential barriers due to the ligant capping molecules.     

CdSe/Cd(x)Zn1-xS core/shell nanocrystals: core morphology and luminescent property

CdSe cores with rod (an aspect ratio of 1.8, d-5 nm) and spherical (an aspect ratio of 1, d-5 nm) morphologies were fabricated by two kinds of organic approaches through adjusting growth processes. Because of large difference of size and morphology, two kinds of cores revealed different absorption spectra. However, these cores exhibited almost same photoluminescence (PL) spectra with a red-emitting PL peak of around 625 nm. This is ascribed that they have a similar size in diameter. A graded Cd(x)Zn1-xS shell of larger band gap was grown around CdSe rods and spheres using oleic acid as a capping agent. Based on the growth kinetics of CdS and ZnS, interfacial segregation was created to preferentially deposit CdS near the core, providing relaxation of the strain at the core/shell interface. For spherical CdSe cores, the homogeneous deposition of the Cd(x)Zn1-xS shell created spherical core/shell nanocrystals (NCs) with a size of 7.1 nm in diameter. In the case of using CdSe cores with rod morphology, the anisotropic aggregation behaviors of CdS monomers on CdSe rods led to the size (approximately 10 nm in diameter) of spherical CdSe/Cd(x)Zn1-xS core/shell NCs with a small difference to the length of the CdSe rod (approximately 8.9 nm). The resulting spherical core/shell NCs created by the rod and spherical cores exhibited almost same PL peak wavelength (652 and 653 nm for using rod and spherical cores, respectively), high PL efficiency up to 50%, and narrow PL spectra (36 and 28 nm of full with at half maximum of PL spectra for the core/shell NCs with CdSe spheres and rods, respectively). These core/shell NCs provide an opportunity for the study of the evolution of PL properties as the shape of semiconductor NCs.     

Ultrafast electronic relaxation and charge-carrier localization in CdS/CdSe/CdS quantum-dot quantum-well heterostructures

The relaxation and localization times of excited electrons in CdS/CdSe/CdS colloidal quantum wells were measured using subpicosecond spectroscopy. HRTEM analysis and steady-state PL demonstrate a narrow size distribution of 5-6 nm epitaxial crystallites. By monitoring the rise time of the stimulated emission as a function of pump intensity, the relaxation times of the electron from the CdS core into the CdSe well are determined and assigned. Two-component rise times in the stimulated emission are attributed to intraband relaxation of carriers generated directly within the CdSe well (fast component) and charge transfer of core-localized carriers across the CdS/CdSe interface (slow component). This is the first reported observation of simultaneous photon absorption in the core and well of a quantum-dot heterostructure. With increasing pump intensity, the charge-transfer channel between the CdS core CdSe well contributes less to the stimulated emission signal because of filling and saturation of the CdSe well state, making the interfacial charge-transfer component less efficient. The interfacial charge-transfer time of the excited electron was determined from the slow component of the stimulated emission build-up time and is found to have a value of 1.2 ps.     

Highly luminescent blue emitting CdS/ZnS core/shell quantum dots via a single-molecular precursor for shell growth

Highly luminescent blue-emitting CdS/ZnS core/shell quantum dots (QDs) were synthesized in N-oleoylmorpholine by two facile steps: first, the CdS core QDs were prepared via a simple one-pot method involving a direct reaction of Cd precursor cadmium stearate and S precursor S powder in solvent N-oleoylmorpholine; second, ZnS shells were successively overcoated on CdS core through the decomposition of single molecular precursor zinc diethyldithiocarbamate. The thickness of shell was precisely tuned by controlling drip feed speed and amount of shell precursor. The obtained CdS/ZnS core/shell QDs showed the maximum photoluminescent quantum yield of 54.8% and narrow spectra bandwidth, exhibiting high monodispersity, good color purity and long fluorescent lifetimes. The CdS/ZnS core/shell QDs with tunable emission wavelength of 424–470 nm were obtained by controlling the thickness of ZnS shell overgrown on different-sized CdS QDs, which are promising materials for blue light-emitting devices.     

Seeded growth of highly luminescent CdSe/CdS nanoheterostructures with rod and tetrapod morphologies

We have demonstrated that seeded growth of nanocrystals offers a convenient way to design nanoheterostructures with complex shapes and morphologies by changing the crystalline structure of the seed. By using CdSe nanocrystals with wurtzite and zinc blende structure as seeds for growth of CdS nanorods, we synthesized CdSe/CdS heterostructure nanorods and nanotetrapods, respectively. Both of these structures showed excellent luminescent properties, combining high photoluminescence efficiency (approximately 80 and approximately 50% for nanorods and nanotetrapods, correspondingly), giant extinction coefficients (approximately 2 x 10(7) and approximately 1.5 x 10(8) M(-1) cm(-1) at 350 nm for nanorods and nanotetrapods, correspondingly), and efficient energy transfer from the CdS arms into the emitting CdSe core.     

Single‐Mode Lasing from Colloidal Water‐Soluble CdSe/CdS Quantum Dot‐in‐Rods

Core–shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution‐phase synthesis is based on surface‐stabilizing ligands that make them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties poses a major challenge. This work demonstrates how core–shell CdSe/CdS quantum dot‐in‐rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light‐emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot‐in‐rods. Films made from water‐soluble dot‐in‐rods show amplified spontaneous emission (ASE) with a similar threshold (130 μJ/cm²) as the pristine material (115 μJ/cm²). To demonstrate feasibility for lasing applications, self‐assembled microlasers are fabricated via the “coffee‐ring effect” that display single‐mode operation and a very low threshold of ～10 μJ/cm². The performance of these microlasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot‐in‐rods.     

Determination of band offsets in heterostructured colloidal nanorods using scanning tunneling spectroscopy

The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.