Water-soluble CdSe and CdSe/CdS nanocrystals: a greener synthetic route

We report a new green synthetic route of CdSe and core-shell CdSe/CdS nanoparticles (NPs) in aqueous solutions. This route is performed under water-bath temperature, using Se powder as a selenium source to prepare CdSe NPs, and H(2)S generated by the reaction of Na(2)SH(2)SO(4) as a sulfur source to synthesize core-shell CdSe/CdS NPs at 25-35 degrees C. The synthesis time of every step is only 20 min. After illumination with ambient natural light, photoluminescence (PL) intensities of CdSe NPs enhanced up to 100 times. The core-shell CdSe/CdS NPs have stronger photoactive luminescence with quantum yields over 20%. The obtained CdSe NPs exhibit a favorable narrow PL band (FWHM: 50-37 nm) with increasing molar ratio of Cd/Se from 4:1 to 10:1 at pH 9.1 in the crude solution, whereas PL band of corresponding CdSe/CdS NPs is slightly narrower. The emission maxima of nanocrystals can be tuned in a wider range from 492 to 592 nm in water by changing synthesis temperature of CdSe core than those reported previously. The resulting new route is of particular interest as it uses readily-available reagents and simple equipment to synthesize high-quality water-soluble CdSe and CdSe/CdS nanocrystals.     

Aqueous synthesis of CdSe and CdSe/CdS quantum dots with controllable introduction of Se and S sources

A new and convenient route is developed to synthesize CdSe and core-shell CdSe/CdS quantum dots(QDs) in aqueous solution.The gaseous precursors,H2Se and H2S,generated on-line by reducing SeO 3 2à with NaBH 4 and the reaction between Na 2 S and diluted H2SO 4,are used to form high-quality CdSe and CdSe/CdS QDs,respectively.The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield(3% and 20%) and narrow full-width-at-half-maximum(43 nm and 38 nm).The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals,and can be readily extended to the large-scale aqueous synthesis of QDs.     

Composition-tunable Zn(x)Cd(1-x)Se nanocrystals with high luminescence and stability

High-quality Zn(x)Cd(1-x)Se nanocrystals have been successfully prepared at high temperature by incorporating stoichiometric amounts of Zn and Se into pre-prepared CdSe nanocrystals. With increasing Zn content, a composition-tunable emission across most of the visible spectrum has been demonstrated by a systematic blue-shift in emission wavelength. The photoluminescence (PL) properties for the obtained Zn(x)Cd(1-x)Se nanocrystals (PL efficiency of 70-85%, fwhm = 22-30 nm) are comparable to those for the best reported CdSe-based QDs. In particular, they also have good PL properties in the blue spectral range. Moreover, the alloy nanocrystals can retain their high luminescence (PL efficiency of over 40%) when dispersed in aqueous solutions and maintain a symmetric peak shape and spectral position under rigorous experimental conditions. A rapid alloying process was observed at a temperature higher than "alloying point". The mechanism of the high luminescence efficiency and stability of Zn(x)Cd(1-x)Se nanocrystals is explored.     

Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction

Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240 degrees C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy, TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed, some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells.     

Control of photoluminescence properties of CdSe nanocrystals in growth

The photoluminescence (PL) quantum yield (QY) of CdSe nanocrystals during their growth under a given set of initial conditions increases monotonically to a certain maximum value and then decreases gradually. Such a maximum is denoted as a PL "bright point", which does not always overlap with the minimum point of the PL peak width for the same reaction. The experimental results suggest that the existence of the PL bright point is a general phenomenon during the growth of semiconductor nanocrystals and likely is a signature of an optimal surface structure/reconstruction of the nanocrystals grown under a given set of initial conditions. The position of the bright point, the highest PL QY, the types of the bright points (sharp or flat), the sharpness of the PL peak, etc., were all strongly dependent on the initial Cd:Se ratio of the precursors in the solution. A large excess of the selenium precursor, with 5-10 times more selenium precursor than the amount of the cadmium precursor, was found necessary to achieve a high PL QY value and a narrow emission profile. The existence of the PL bright point and the sensitive temporal variation of the PL QY during the growth of semiconductor nanocrystals can explain the unpredictable nature and poor reproducibility of the PL properties of the as-prepared semiconductor nanocrystals observed previously. Furthermore, the knowledge gained in this study enabled us to reproducibly synthesize highly luminescent CdSe nanocrystals through a relatively simple and safe synthetic scheme. In a traditionally weak emission window for CdSe nanocrystals, the orange-red optical window, the PL QY of the as-prepared CdSe nanocrystals reached as high as 85% at room temperature, and the full width at half-maximum of the corresponding PL peak was as narrow as 23 nm, about 65-80 meV depending on the emitting position. The PL properties of the as-prepared CdSe nanocrystals are stable upon aging for at least several months. These as-prepared nanocrystals represent a series of best emitters that are highly efficient, highly pure in emission color, stable, and continuously tunable by simply varying the size of the nanocrystals.     

Size- and shape-controlled synthesis of ZnSe nanocrystals using SeO2 as selenium precursor

Here we report a low-cost and "green" phosphine-free route for the size- and shape-controlled synthesis of high-quality zinc blende (cubic) ZnSe nanocrystals. To avoid the use of expensive and toxic solvents such as trioctylphosphine (TOP) or tributylphosphine (TBP), SeO(2) was dispersed in 1-octadecene (ODE) as a chalcogen precursor. It has been found that the temperature and the surface ligand influenced the nucleation, the reaction speed and the formation of different shapes. Absorption spectroscopy, fluorescence spectroscopy, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used for the characterization of the as-synthesized ZnSe nanocrystals. The size-dependent photoluminescence (PL) range of the as-prepared ZnSe nanocrystals was between 390 and 450 nm, with the PL full width at half-maximum (FWHM) well controlled between 14 and 18 nm and PL quantum yields reached up to 40% at room temperature. Moreover, this new selenium precursor can be used to form tetrapod-shaped ZnSe nanocrystals when zinc acetylacetonate was introduced as the zinc precursor with a one-pot method.     

A new two-phase route to high-quality CdS nanocrystals

A new two-phase route has been developed to synthesize high-quality CdS nanocrystals with a narrow size distribution and a high photoluminescence (PL) quantum yield (QY). In the two-phase system, toluene and water were used as separate solvents for cadmium myristate (CdM2) and thiourea, which served as cadmium source and sulfur source, respectively, and oleic acid (OA) was used as a ligand for stabilizing the nanocrystals. The reactions were completed in the heated autoclaves. The initial Cd/S molar ratio of the precursors and the reaction temperature were found to be factors that affected the growth of nanocrystals. Furthermore, a seeding-growth technique was developed to synthesize CdS nanocrystals of different sizes, which exhibit PL peaks with quite similar full width at half-maximum (FWHM) values compared to those of the initial nanocrystal seeds in all cases.     

Size/shape-controlled synthesis of colloidal CdSe quantum disks: ligand and temperature effects

Size/shape-controlled colloidal CdSe quantum disks with zinc-blende (cubic) crystal structure were synthesized using air-stable and generic starting materials. The colloidal CdSe quantum disks were approximately square, and their lateral dimensions were varied between 20 and 100 nm with the thickness controlled between 1 and 3 nm, which resulted in sharp and blue-shifted UV-vis and PL peaks due to one-dimensional quantum confinement. The quantum disks were grown with either <001> or <111> direction, polar directions in the single crystalline disks, as the short axis, and both basal planes were terminated with Cd ions. These surface Cd ions were passivated with negatively charged fatty acid ligands to neutralize the net positive charges caused by the excess monolayer of Cd ions. The coordination of the Cd ions and carboxylate groups further enabled the close-packing monolayer of fatty acid ligands on each basal plane. The close packing of the hydrocarbon chains of fatty acids dictated the up temperature limit for synthesis of the colloidal quantum disks, and the low temperature limit was found to be related to the reactivity of the starting materials. Overall, a high Cd to Se precursor ratio, negative-charged fatty acid ligands with a long hydrocarbon chain, and a proper temperature range (approximately between 140 and 250 °C) were found to be needed for successful synthesis of the colloidal CdSe quantum disks.     

Comparative study on coating CdSe nanocrystals with surfactants

We have synthesized CdSe nanocrystals (NCs) in sizes from 2.2 to 5.1 nm passivated with hydrophobic trioctylphosphine oxide (TOPO) in combination trioctylphosphine (TOP) or tributylphosphine (TBP) to obtain particles of the type CdSe/TOPO/TOP or CdSe/TOPO/TBP. These NCs were then dispersed in aqueous solution of ionic or non-ionic surfactants (such as stearate, oleic acid, Tween) using a biphase (water and chloroform or hexane) transfer method. It is found that both the structure of the surfactant and the native surface of the ligand govern the coating of the NCs with surfactants. More specifically, the hydrophobicity-hydrophilicity balance of the surfactant regulates the coating efficacy, thereby transferring the NC from the organic to the aqueous phase. The type of ligand on the NCs and the kind of coating surfactant also affect photoluminescence (PL). The ratio of PL and absorbance unit (defined as PL per 0.1 AU) was implemented as a tool to monitor changes in PL intensity and wavelength as a function of size, coatings and surface defects. Finally, the distribution of CdSe nanocrystals between pseudophases in cloud point extraction was discussed based on experimental results. It was concluded that the size of CdSe nanocrystal present in an appropriate pseudophase is correlated with the way in which the non-ionic surfactant coats CdSe nanocrystals.

Microwave synthesis of CdSe and CdTe nanocrystals in nonabsorbing alkanes

Controlling nanomaterial growth via the "specific microwave effect" can be achieved by selective heating of the chalcogenide precursor. The high polarizability of the precursor allows instantaneous activation and subsequent nucleation leading to the synthesis of CdSe and CdTe in nonmicrowave absorbing alkane solvents. Regardless of the desired size, narrow dispersity nanocrystals can be isolated in less than 3 min with high quantum efficiencies and elliptical morphologies. The reaction does not require a high temperature injection step, and the alkane solvent can be easily removed. In addition, batch-to-batch variance in size is 4.2 +/- 0.14 nm for 10 repeat experimental runs. The use of a stopped-flow reactor allows near continuous automation of the process leading to potential industrial benefits.     

Planar polarized light emission from CdSe nanoparticle clusters

This paper describes synthesis and optical properties of planar clusters of CdSe nanocrystals. The clusters emit linearly polarized light in the plane of the cluster. The emission wavelength of the clusters can be adjusted between 568 and 639 nm with the size of the CdSe nanocrystals. Planar CdSe microclusters were synthesized by reaction of trioctylphosphine oxide-coated CdSe/CdS nanocrystals with 3-aminopropylsilyl-modified Ca(2)Nb(3)O(10) nanosheets in THF. The clusters are 3.92 +/- 1.18 mum length/width and 91 +/- 37 nm thickness, and they consist of alternating layers of Ca(2)Nb(3)O(10) to which CdSe nanocrystals are attached with densities of 5300 +/-310 particles per side of a single Ca(2)Nb(3)O(10) sheet. The chemical inertness of the clusters in coordinating solvents suggests covalent interactions between the aminopropyl groups and CdSe nanocrystals. Upon excitation at lambda(exc) = 400 nm, the clusters emit green (568 nm), orange (589 nm), or red (639 nm) light, depending on the size of the CdSe crystals. The light is emitted preferentially in the cluster plane and it is linearly polarized along the cluster edges. Combined fluorescence microscopy and atomic force microscopy reveal that the directional emission efficiency depends linearly on the thickness of the clusters, which varies between 70 and 180 nm. The ability to manipulate the direction and polarization of the photoemission of CdSe nanoparticles via assembly into 2D structures is of interest for applications of these and similar structures in advanced optical materials and devices.     

Effect of reaction media on the growth and photoluminescence of colloidal CdSe nanocrystals

Using cadium oxide (CdO) as the Cd precursor and tri-n-octylphosphine selenide (TOPSe) as the Se source, TOP-capped and TOP/tri-n-octylphosphine oxide (TOPO)-capped CdSe nanocrystals were synthesized without the use of an acid. The synthetic approach involved the addition of a TOPSe/TOP solution into a CdO/TOP solution with or without TOPO at one temperature and subsequent growth at a lower temperature. The temporal evolution of the optical properties, namely, absorption and luminescence, of the growing nanocrystals was monitored in detail. A comprehensive examination on the control of the photoluminescence (PL) properties was performed by systematically varying the TOP/TOPO weight ratio of the reaction media. Surprisingly, a rational choice of 100% TOP or 80% TOP was found to produce "quality" nanocrystals when monitored under the present experimental conditions and growth-time scale. The term "quality" is mainly based on the sharp features and rich substructure exhibited in the absorption spectra of the growing nanocrystals, as well as the sharp features in the emission spectra with narrow full width at half-maximum (fwhm). There are two distinguishable stages of growth: an early stage (<5 min) and a later stage. TOP plays a major role in the control of a slow growth rate in the early stage, while TOPO controls slow growth in the later stage. The optical sensitivity of the growing nanocrystals when dispersed in nonpolar or polar solvents was studied, including two size-dependent parameters, namely, the solvent sensitivity (PL intensity) and nonresonant Stokes shift (NRSS). The insights gained from the present study enable a synthetic approach in which high-quality CdSe nanocrystals are achieved with high synthetic reproducibility.     

Temperature-sensitive photoluminescence of CdSe quantum dot clusters

The formation of narrow size dispersed and nanometer size aggregates (clusters) of cadmium selenide (CdSe) quantum dots (QDs) and their temperature-sensitive photoluminescence (PL) spectral properties close to room temperature (298 K) are discussed. CdSe QDs formed stable clusters with an average diameter of approximately 27 nm in the absence of coordinating solvents. Using transmission electron microscopy (TEM) imaging, we identified the association of individual QDs with 2-5 nm diameters into clusters of uniform size. A suspension of these clusters in different solvents exhibited reversible PL intensity changes and PL spectral shifts which were correlated with temperature. Although the PL intensity of CdSe QDs encapsulated in host matrixes and the solid state showed a response to temperature under cryogenic conditions, the current work identified for the first time QD clusters showing temperature-sensitive PL intensity variations and spectral shifts at moderate temperatures above room temperature. Temperature-sensitive reversible PL changes of clusters are discussed with respect to reversible thermal trapping of electrons at inter-QD interfaces and dipole-dipole interactions in clusters. Reversible luminescence intensity variations and spectral shifts of QD clusters show the potential for developing sensors based on QD nanoscale assemblies.     

Synthesis of Cysteine-Capped Zn(x)Cd(1)(-)(x)Se alloyed quantum dots emitting in the blue-green spectral range

Alloyed ZnxCd1-xSe quantum dots (QDs) have been successfully prepared at low temperatures by reacting a mixture of Cd(ClO4)2 and Zn(ClO4)2 with NaHSe using cysteine as a surface-stabilizing agent. The photoluminescence (PL) spectra of the alloyed QDs are determined on the basis of the Zn2+/Cd2+ molar ratio, reaction pH, intrinsic Zn2+and Cd2+ reactivities toward NaHSe, concentration of NaHSe, and the kind of thiols. A systematic blue shift in emission wavelength of the alloyed QDs was found with the increase in the Zn mole fraction. This result provides clear evidence of the formation of ZnxCd1-xSe QDs by the simultaneous reaction of Zn2+ and Cd2+ with NaHSe, rather than the formation of separate CdSe and ZnSe nanocrystals or core-shell structure CdSe/ZnSe nanocrystals. The size and inner structure of these QDs are also corroborated by using high-resolution transmission electron microscopy and X-ray powder diffraction. To further understand the formation mechanism, the growth kinetics of Zn0.99Cd0.01Se was studied by measuring the PL spectra at different growth intervals. The results demonstrated that, in the initial stage of growth, Zn0.99Cd0.01Se has a structure with a Cd-rich core and a Zn-rich shell. The post-preparative irradiation of these QDs improved their PL properties, resulting in stronger emission.     

Chemical and optical properties of CdSe and CdSe/ZnS nanocrystals investigated using high-performance liquid chromatography

We apply a variety of characterization tools, including dynamic light scattering (DLS), transmission electron microscopy (TEM), high-resolution size-exclusion chromatography (HRSEC), and X-ray fluorescence (XRF), to study CdSe and CdSe/ZnS semiconductor nanocrystals of various sizes. We compare the size monodispersity, composition, and optical properties such as absorbance, photoluminescence (PL), and photoluminescence excitation of samples synthesized by high-temperature organometallic decomposition methods to CdSe clusters synthesized in our laboratory using a room-temperature metathesis from ionic precursors in coordinating solvents. DLS revealed considerable aggregation in all the conventionally synthesized samples, while TEM showed significant size and shape polydispersity in the core/shell CdSe/ZnS nanoparticles. We demonstrate how HRSEC can be used to explore size and shape polydispersity in semiconductor nanocrystals by measurement of the spectral homogeneity of the PL and PLE of spectra obtained within cluster elution peaks observed by HRSEC. Using HRSEC, we show that size fractionation by solvent/nonsolvent precipitation is only partially effective in size selection and that discrete size populations are present in each fraction. HRSEC shows that our synthesis yields a single-size, blue-emitting, homogeneous population whose absorbance and PL correspond to those of the smallest-size fraction made by conventional synthesis. This suggests that especially stable discrete sizes are favored in both synthetic methods.     

无膦方法合成高质量CdSe纳米晶及其光学性质

以合成的十碳酸镉作为Cd前驱体, 十八烯作为单质硒溶剂, 并添加十八胺作为活性剂, 在无三丁基膦或三辛基膦参与的条件下, 以较低温度制备了具有闪锌矿结构的高质量的CdSe纳米晶. 利用吸收光谱、荧光光谱(PL)、X射线衍射(XRD)、透射电镜(TEM)对不同反应时间得到的CdSe纳米晶进行形貌和光谱性质表征. 实验结果表明, 采用该无膦法只需调控反应时间就可得到粒径均一、分散性好的CdSe纳米晶, 其荧光波长可覆盖470-630 nm的可见光区, 而荧光峰半高宽则始终保持在24-30 nm之间并具有较高的荧光量子产率(535 nm处大于60%). 最后, 对CdSe纳米晶量子产率随反应时间变化的原因进行了分析.     

Influence of Cd Content and Se Doping on the Formation of CdSe Nanocrystals in Silica Xerogels: A SAXS Study

Small-angle X-ray scattering (SAXS) experiments were carried out to characterize the structure of the composite formed by CdSe nanocrystals embedded in a popous silica matrix (silica xerogels containing Cd with formamide addition and ultrasound treatment). SAXS results from samples before Se diffusion indicate the presence of heterogeneities with a bimodal size distribution which was associated to the existence of mesopores (pores of several hundred Å) immersed in a nanoporous matrix (characteristic pore radii of 20–30 Å). The diffusion of Se induces the nucleation and growth of CdSe nanocrystals. The average size of the nanocrystals increases with Cd content. Higher Se doses promote the formation of larger nanocrystals (radius of gyration of ∼30 to 50 Å). Anomalous scattering results confirm the existence of Se aggregation associated with CdSe nanocrystal formation and suggest that only partial segregation of Cd and Se occurs.     

Role of surface modification of colloidal CdSe quantum dots on the properties of hybrid organic–inorganic nanocomposites

In this work, tri-octyl phosphine/tri-octyl phosphine oxide (TOPO)-capped cadmium selenide (CdSe) quantum dots (QDs) of varied sizes (5–9 nm), prepared by varying the input Cd:Se precursor ratio using chemical route, were dispersed in conducting polymer matrices viz. poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly(3-hexylthiophene) (P3HT). By using a binary solvent mixture (pyridine–chloroform), homogeneous dispersion of CdSe nanocrystals in polymers (MEH-PPV, P3HT) could be realized. The properties of the resulting dispersions could be tailored by the composition and concentration of QDs in polymer. The emission and structural properties of polymer–CdSe nanocomposites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. An effective quenching of photoluminescence emission in the polymer nanocomposite was observed for smaller CdSe quantum dots (size ∼6 nm) as compared to larger CdSe quantum dots (size ∼9 nm), thus ensuring efficient charge transfer process across the polymer–CdSe interface in the former case. The incomplete quenching, particularly for MEH-PPV:CdSe nanocomposites, could be as a result of insufficient coverage of polymers on the surface of CdSe nanocrystallites, mainly due to phase segregation for TOPO-stripped CdSe nanocrystallites. The superior morphology and optical properties of polymer nanocomposite (P3HT:CdSe QDs) could play a pivotal role for the realization of effective charge separation and transport in hybrid solar cells.     

Preparation and application of L-cysteine-modified CdSe/CdS core/shell nanocrystals as a novel fluorescence probe for detection of nucleic acid

The water-soluble L-cysteine-modified CdSe/CdS core/shell nanocrystals (expressed as CdSe/CdS/Cys nanocrystals) have been synthesized in aqueous by using L-cysteine as stabilizer. The size, shape, component and spectral property of CdSe/CdS/Cys nanocrystals were characterized by high-resolution transmission electron microscope (HRTEM), energy dispersive X-ray fluorescence (EDX), infrared spectrum (IR) and photoluminescence (PL). The results showed that the spherical CdSe/CdS/Cys nanocrystals with an average diameter of 2.3 nm have favorable fluorescent property, theirs photostability and fluorescence intensity are enhanced greatly after overcoating with CdS. The cysteine modified on the surface of core/shell CdSe/CdS nanocrystals renders the nanocrystals water-soluble and biocompatible. Based on the fluorescence quenching of the nanocrystals in the presence of calf thymus deoxyribonucleic acid (ct-DNA), a fluorescence quenching method has been developed for the determination of ct-DNA by using the nanocrystals as a novel fluorescence probe. The pH value of the system was selected at pH 7.4, with excitation and emission wavelength at 380 and 522 nm, respectively. Under the optimal conditions, the fluorescence quenching intensity of the system is linear with the concentration of ct-DNA in the range of 0.1-3.5 microg/mL (r=0.9987). The detection limit is 0.06 microg/mL. And two synthetic samples were analyzed satisfactorily.