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 stable tricolor Zn(1-x)Cd(x)Se core/multishell nanocrystals via a facile phosphine-free colloidal method

Here we report a new "green" method to synthesize Zn(1-x)Cd(x)Se (x = 0-1) and stable red-green-blue tricolor Zn(1-x)Cd(x)Se core/shell nanocrystals using only low cost, phosphine-free and environmentally friendly reagents. The first excitonic absorption peak and photoluminescence (PL) position of the Zn(1-x)Cd(x)Se nanocrystals (the value of x is in the range 0.005-0.2) can be fixed to any position in the range 456-540 nm. There is no red or blue shift in the entire reaction process. Three similar sizes of alloyed Zn(1-x)Cd(x)Se nanocrystals with blue, green, and yellow emissions were successfully selected as cores to synthesize high quality blue, green, and red core/shell nanocrystal emitters. For the synthesis of core/shell nanocrystals with a high quantum yield (QY) and stability, the selection of shell materials has been proven to be very important. Therefore, alternative protocols have been used to optimize thick shell growth. ZnSe/ZnSe(x)S(1-x) and CdS/Zn(1-x)Cd(x)S have been found as an excellent middle multishell to overcoat between the alloyed Zn(1-x)Cd(x)Se core and ZnS outshell. The QYs of the as-synthesized core/shell alloyed Zn(1-x)Cd(x)Se nanocrystals can reach 40-75%. The Cd content is reduced to less than 0.1% for Zn(1 -x)Cd(x)Se core/shell nanocrystals with emissions in the range 456-540 nm. More than 15 g of high quality Zn(1-x)Cd(x)Se core/shell nanocrystals were prepared successfully in a large scale, one-pot reaction. Importantly, the emissions of such thick multishell nanocrystals are not susceptible to ligand loss and stability in various physiological conditions.     

利用离子型Cd源制备ZnSe/CdSe核-壳结构纳米晶

在有机相体系中利用ZnSe前驱体纳米晶制备过程中的富Se环境,以引入Cd2+的方式在相对温和的环境下通过控制Cd2+离子的加入量及调节反应时间,成功制备了ZnSe/CdSe核-壳复合结构纳米晶.利用X射线衍射(XRD)、透射电镜(TEM)、紫外-可见吸收光谱(UV-vis)和荧光光谱(FL)对其结构形貌以及光学性质进行表征和分析的结果表明,CdSe以外延生长的方式包覆在ZnSe纳米晶表面从而形成具有良好结晶性的核-壳复合结构,其荧光发射始终保持良好单色性,同时实现了在500～620nm可见光范围内的连续可调.     

Highly luminescent CdSe/Cd(x)Zn(1-x)S quantum dots coated with thickness-controlled SiO2 shell through silanization

A silanization technique of hydrophobic quantum dots (QDs) was applied to SiO(2)-coated CdSe/Cd(x)Zn(1-x)S QDs to precisely control the SiO(2) shell thickness and retain the original high photoluminescence (PL) properties of the QDs. Hydrophobic CdSe/Cd(x)Zn(1-x)S core-shell QDs with PL peak wavelengths of 600 and 652 nm were prepared by a facile organic route by using oleic acid (OA) as a capping agent. The QDs were silanized by using partially hydrolyzed tetraethyl orthosilicate by replacing surface OA. These silanized QDs were subsequently encapsulated in a SiO(2) shell by a reverse micelles synthesis. The silanization plays an important role for the QDs to be coated with a homogeneous SiO(2) shell and retain a high PL efficiency in water. Transmission electron microscopy observation shows that the shells are 1-9 nm with final particle sizes of 10-25 nm, depending on the initial QD size. In the case of short reaction time (6 h), the QDs were coated with a very thin SiO(2) layer because no visible SiO(2) shell was observed but transferred into the water phase. The silica coating does not change the PL peak wavelength of the QDs. The full width at half-maximum of PL was decreased 4 nm after coating for QDs emitting at both 600 and 652 nm. The PL efficiency of the SiO(2)-coated is up to 40%, mainly determined by the initial PL efficiency of the underlying CdSe/Cd(x)Zn(1-x)S QDs.     

Alloyed Zn(x)Cd(1-x)S nanocrystals with highly narrow luminescence spectral width

High-quality alloyed Zn(x)Cd(1-x)S nanocrystals have been synthesized at high temperature by the reaction of a mixture of CdO- and ZnO-oleic acid complexes with sulfur in the noncoordinating solvent octadecene system. A series of monodisperse wurtzite Zn(x)Cd(1-x)S (x = 0.10, 0.25, 0.36, 0.53) nanocrystals were obtained with corresponding particle radii of 4.0, 3.2, 2.9, and 2.4 nm, respectively. With the increase of the Zn content, their photoluminescence (PL) spectra blue-shift systematically across the visible spectrum from 474 to 391 nm, indicating the formation of the alloyed nanocrystals. The alloy structure is also supported by the characteristic X-ray diffraction (XRD) patterns of these nanoalloys with different Zn mole fractions, in which their diffraction peaks systematically shift to larger angles as the Zn content increases. The lattice parameter c measured from XRD patterns decreases linearly with the increase of Zn content. This trend is consistent with Vegard's law, which further confirms the formation of homogeneous nanoalloys. These monodisperse wurtzite Zn(x)Cd(1-x)S nanoalloys possess superior optical properties with PL quantum yields of 25-50%, especially the extremely narrow room-temperature emission spectral width (full width at half-maximum, fwhm) of 14-18 nm. The obtained narrow spectral width stems from the uniform size and shape distribution, the high composition homogeneity, and the relatively large particle radius, which is close to or somewhat larger than the exciton Bohr radius. The process by which the initial structure with random spatial composition fluctuations turns into an alloy (solid solution) with homogeneous composition is clearly demonstrated by the temporal evolution of the PL spectra during the annealing progress.     

Synthesis of CdSe, ZnSe, and ZnxCd1-xSe nanocrystals and their silica sheathed core/shell structures

Uniform ZnxCd1-xSe nanocrystals have been prepared at the artificially designed water-oil interface using Na2SeO3, Cd(NO3)2, and Zn(NO3)2 as precursors. The chemical composition and band gap of the ZnxCd1-xSe nanocrystals can be adjusted via different combinations of source material. The coating of a SiO2 shell could transform the hydrophobic particles into hydrophilic particles. An advantage of this method is that a water phase could be added to the oleic acid (OLEA) synthesis system, which could be extended to make the synthesis of various nanocrystals more simple and flexible.     

Bimodal bond-length distributions in cobalt-doped CdSe, ZnSe, and Cd1-xZnxSe quantum dots

Electronic absorption spectroscopy has been used to study changes in Co2+ ligand-field parameters as a function of alloy composition in Co2+-doped Cd(1-x)Zn(x)Se nanocrystals. A shift in the energy of the 4T1(P) excited-state with alloy composition is observed. Analysis reveals that Co2+-Se2- bond lengths change relatively little as the host is varied continuously from CdSe to ZnSe, generating a large difference between microscopic and average cation-anion bond lengths in Co2+-doped CdSe nanocrystals but not in Co2+-doped ZnSe nanocrystals. The bimodal bond-length distributions observed here are shown to cause a diameter-dependent enthalpic destabilization of doped semiconductor nanocrystals.     

One-pot noninjection synthesis of Cu-doped Zn(x)Cd(1-x)S nanocrystals with emission color tunable over entire visible spectrum

Unlike Mn doped quantum dots (d-dots), the emission color of Cu dopant in Cu d-dots is dependent on the nature, size, and composition of host nanocrystals (NCs). The tunable Cu dopant emission has been achieved via tuning the particle size of host NCs in previous reports. In this paper, for the first time we doped Cu impurity in Zn(x)Cd(1-x)S alloyed NCs and tuned the dopant emission in the whole visible spectrum via variation of the stoichiometric ratio of Zn/Cd precursors in the host Zn(x)Cd(1-x)S alloyed NCs. A facile noninjection and low cost approach for the synthesis of Cu:Zn(x)Cd(1-x)S d-dots was reported. The optical properties and structure of the obtained Cu:Zn(x)Cd(1-x)S d-dots have been characterized by UV-vis spectroscopy, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The influences of various experimental variables, including Zn/Cd ratio, reaction temperature, and Cu dopant concentration, on the optical properties of Cu dopant emission have been systematically investigated. The as-prepared Cu:Zn(x)Cd(1-x)S d-dots did show PL emission but with quite low quantum yield (QY) (typically below 6%). With the deposition of ZnS shell around the Cu:Zn(x)Cd(1-x)S core NCs, the PL QY increased substantially with a maximum value of 65%. More importantly, the high PL QY can be preserved when the initial oil-soluble d-dots were transferred into aqueous media via ligand replacement by mercaptoundeconic acid. In addition, these d-dots have thermal stability up to 250 °C.     

Alloyed (ZnSe)(x)(CuInSe2)(1-x) and CuInSe(x)S(2-x) nanocrystals with a monophase zinc blende structure over the entire composition range

Metastable zinc blende CuInSe(2) nanocrystals were synthesized by a hot-injection approach. It was found that the lattice mismatches between zinc blende CuInSe(2) and ZnSe as well as CuInSe(2) and CuInS(2) are only 2.0% and 4.6%, respectively. Thus, alloyed (ZnSe)(x)(CuInSe(2))(1-x) and CuInSe(x)S(2-x) nanocrystals with a zinc blende structure have been successfully synthesized over the entire composition range, and the band gaps of alloys can be tuned in the range from 2.82 to 0.96 eV and 1.43 to 0.98 eV, respectively. These alloyed (ZnSe)(x)(CuInSe(2))(1-x) and CuInSe(x)S(2-x) nanocrystals with a broad tunable band gap have a high potential for photovoltaic and photocatalytic applications.     

Preparation of CdSe/ZnSe Core-shell Nanocrystal in One-step Reaction

IntroductionSemiconductor nanocrystals show strong size-de-pendent properties when their size is similar to or smal-ler than the excition Bohr radius of the bulk materialsand quantum confinement occurs for the space-con-fined motion of the electrons and holes in the nano-re-gion of materials[1—5].Because of the excellent opticaland electronic properties,semiconductor nanocrystalsare currently being investigated as emitting materials forthin-film light-emitting devices(LED)[6,7],low-thresh-ol…     

ZnxCd1-xS∶Ag纳米晶的合成及其光学性质研究

以巯基丙酸(MPA)为稳定剂, 利用共沉淀法制备了水溶性的Ag掺杂的ZnxCd1-xS合金型纳米晶. Ag掺杂后ZnxCd1-xS纳米晶产生新的发射峰, 并且发光效率得到了有效提高. 通过改变纳米粒子中Zn/Cd比例可有效地调控ZnxCd1-xS∶Ag纳米晶的吸收带隙宽度, 同时可以在425～603 nm之间实现对ZnxCd1-xS∶Ag纳米晶发射峰位的连续调控.     

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.     

A Novel One‐Pot Synthesis of UV‐Blue ZnSe Nanocrystals in Aqueous Solution

Described herein is a novel one‐pot aqueous synthesis of ZnSe nanocrystals has featuring the utilization of Na₂SeO₃ and Zn(AC)₂×2H₂O as Se and Zn source, glutathione (GSH) as stabilizing agent and reducing agent. By this approach, the UV‐blue ZnSe QDs with quantum yield (QYs) up to 19% have been synthesized with a molar ratio of Se/Zn/GSH at 1:4:8.5 under aqueous conditions at 110 °C. XRD and TEM show the ZnSe QDs are zinc cubic structure particles with an average diameter of 3–5 nm.     

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.     

Kinetic analysis for formation of Cd1-xZnxSe solid-solution nanocrystals

Kinetic analysis on the nanocrystal solid-solution formation was performed by heat treating CdSe/ZnSe core/shell nanocrystals, synthesized via a typical TOP/TOPO approach, at different temperatures for different time periods. X-ray diffraction (XRD) peak shifts in Cd1-xZnxSe cores according to the solid-solution treatments were monitored and used for the estimation of the lattice parameter change. The degree of solid-solution formation was determined considering the compositional variation in Cd1-xZnxSe cores, which was obtained from the Vegard's law. The degree of solid-solution formation (x) was applied to Jander analysis, and an Arrhenius-type plot was produced using the slopes of Jander plots. The activation energy for solid-solution formation was determined as approximately 152 kJ/mol, which evidently indicates that the diffusion of Zn2+ ions in the CdSe-ZnSe system is the governing mechanism for the Cd1-xZnxSe solid-solution formation. The Jander equation to predict the solid-solution formation kinetics for the CdSe/ZnSe core/shell systems was completed using the reaction rate constant (k).     

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.     

A new route to zinc-blende CdSe nanocrystals: mechanism and synthesis

We report the possible mechanism of forming of CdSe nanocrystals in the high boiling point solvents with long alkane chains and a novel Non-TOP-Based route to zinc-blende CdSe nanocrystals. A new mechanism shows that there exits a redox reaction in the long alkane chain solvents: Se is reduced to H2Se gas; at the same time, the long alkane chains are oxidated to alkene chains; then, the Cd complex reacts with H2Se to form CdSe nanocrystals. Possible chemical reaction equations involved in the process of forming the CdSe nanocrystals have been discussed. The alkene chain and H2Se were detected respectively by a series of experiments to support the new mechanism. Under the guidance of this mechanism, we have developed a much cheaper and greener Non-TOP-Based route for the synthesis of a size series of high-quality zinc-blende (cubic) CdSe nanocrystals. Low-cost, green, and environmentally friendlier reagents are used, without use of expensive solvents such as trioctylphosphine (TOP) or tributylphosphine (TBP). The new route enables us to achieve high-quality CdSe nanocrystals with sharp ultraviolet and visible (UV-vis) absorption peaks, controllable size (2.0-5.0 nm), bright photoluminescence (PL), narrow PL full width of half-maximum (fwhm) (29-48 nm), and high PL quantum yield (up to 60%) without any size sorting.     

From (Cd2Se2)(pa) (pa=propylamine) hybrid precursors to various CdSe nanostructures: structural evolution and optical properties

Nanostructural evolution of II-VI semiconductors emerges as a hot research field because it offers a novel route for the synthesis of functional semiconductor nanomaterials. Here, we report a "top-down" process of structural evolution from two dimensional Cd(2)Se(2)(pa) hybrid structures to zero- or one- dimensional CdSe nanostructures. We firstly synthesized the Cd(2)Se(2)(pa) hybrid by a facile solvothermal reaction and determined the hybrid crystal structure through the Rietveld refinement based on the PXRD data. The (Cd(2)Se(2))(pa) hybrid consists of [Cd(2)Se(2)] slabs sandwiched by coordinated n-propylamine layers. Then, we used this hybrid as a precursor to explore the "top-down" fabrication process of structural evolution from two dimensional layered structures to zero- or one- dimensional nanocrystals. It has been shown that various CdSe nanostructures including lamellar structures, nanoparticles, nanowires, and nanorods can be obtained through the pyrolysis of (Cd(2)Se(2))(pa) hybrid precursor under different conditions. It is worth mentioning that the oriented attachment of nanoparticles has been observed during the pyrolysis process. Additionally, the variation of optical properties with structural evolution has been investigated in detail.     

功能化ZnxCd1-xS纳米晶的制备及其光学性能的研究

以氯化锌、氯化镉和硫化钠为原料，采用巯基乙醇为表面修饰剂，H₂O/DMF为溶剂，制得未团聚且分散均匀的纳米晶溶液，同时得到表面富含羟基基团不同锌镉组成的复合Zn_xCd_1-xS纳米晶。利用FTIR、EDS、UV-Vis、XRD、荧光光谱(PL)考察了复合纳米晶结构和光学性能的关系，并利用紫外灯摄像技术对Zn_xCd_1-xS纳米晶的光致发光性能进行     

Synthesis of Cd-free water-soluble ZnSe(1-x)Te(x) nanocrystals with high luminescence in the blue region

Cd-free core-shell nanocrystals (ZnSe(1-x)Te(x)/ZnS, 0相似文献