Synthesis of quasi-two-dimensional colloidal cadmium selenide nanoparticles and formation of sulfide monolayer on their surfaces

We have carried out a solution synthesis of quasi-two-dimensional CdSe nanoparticles (nanoplates) using oleic acid as a stabilizer. Three populations of CdSe nanoplates with different thicknesses quantified by one CdSe monolayer were obtained. The absorption spectra of CdSe nanoplates exhibit narrow excitonic bands (with bandwidths of ～60 meV), whose positions are determined by the nanoplate thickness and are 461, 503, and 551 nm for the three populations. We have systematically studied how formation of sulfide monolayer on the surface of CdSe nanoplates affects their optical properties. The red shift and significant broadening of absorption bands was shown.     

Surface‐State‐Mediated Charge‐Transfer Dynamics in CdTe/CdSe Core–Shell Quantum Dots

Herein, we report the synthesis of aqueous CdTe/CdSe type‐II core–shell quantum dots (QDs) in which 3‐mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core–shell QDs are characterized by X‐ray diffraction (XRD), high‐resolution transmission electron microscopy (HR‐TEM), steady‐state absorption, and emission spectroscopy. A red shift in the steady‐state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR‐TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time‐resolved emission studies, we observe that the average emission lifetime (τ_av) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τ_av=12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type‐II CdTe/CdSe core–shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge‐transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core–shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core–shell QDs.     

Preparation and Characterization of CdTe Nanoparticles in Zirconia Films Prepared by the Sol Gel Method

Two methods for the synthesis of CdTe nanoparticles in zirconia sol-gel films are demonstrated. The nanoparticles were obtained by chemical reduction of Te(IV) using reducing agent (hydrazine) or tin chloride. Particle sizes ranging from 6 to 20 nm in diameter could be prepared by varying the experimetal parameters. The size and crystalline structure of the particles were characterized by optical absorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The film morphology was characterized by scanning force microscopy.The film obtained by SnCl₂ method is smooth and homogeneous. The dense structure of CdTe nanoparticles of a few nm in diameter is revealed. The films prepared with hydrazine are porous as a result of evolution of the decomposition gaseous products during the reduction.Advantages and disadvantages of the two methods are discussed.     

Experimental determination of the absorption cross-section and molar extinction coefficient of CdSe and CdTe nanowires

Absorption cross-sections and corresponding molar extinction coefficients of solution-based CdSe and CdTe nanowires (NWs) are determined. Chemically grown semiconductor NWs are made via a recently developed solution-liquid-solid (SLS) synthesis, employing low melting Au/Bi bimetallic nanoparticle "catalysts" to induce one-dimensional (1D) growth. Resulting wires are highly crystalline and have diameters between 5 and 12 nm as well as lengths exceeding 10 microm. Narrow diameters, below twice the corresponding bulk exciton Bohr radius of each material, place CdSe and CdTe NWs within their respective intermediate to weak confinement regimes. Supporting this are solution linear absorption spectra of NW ensembles showing blue shifts relative to the bulk band gap as well as structure at higher energies. In the case of CdSe, the wires exhibit band edge emission as well as strong absorption/emission polarization anisotropies at the ensemble and single-wire levels. Analogous photocurrent polarization anisotropies have been measured in recently developed CdSe NW photodetectors. To further support fundamental NW optical/electrical studies as well as to promote their use in device applications, experimental absorption cross-sections are determined using correlated transmission electron microscopy, UV/visible extinction spectroscopy, and inductively coupled plasma atomic emission spectroscopy. Measured CdSe NW cross-sections for 1 microm long wires (diameters, 6-42 nm) range from 6.93 x 10(-13) to 3.91 x 10(-11) cm2 at the band edge (692-715 nm, 1.73-1.79 eV) and between 3.38 x 10(-12) and 5.50 x 10(-11) cm2 at 488 nm (2.54 eV). Similar values are obtained for 1 microm long CdTe NWs (diameters, 7.5-11.5 nm) ranging from 4.32 x 10(-13) to 5.10 x 10(-12) cm2 at the band edge (689-752 nm, 1.65-1.80 eV) and between 1.80 x 10(-12) and 1.99 x 10(-11) cm2 at 2.54 eV. These numbers compare well with previous theoretical estimates of CdSe/CdTe NW cross-sections far to the blue of the band edge, having order of magnitude values of 1.0 x 10(-11) cm2 at 488 nm. In all cases, experimental NW absorption cross-sections are 4-5 orders of magnitude larger than those for corresponding colloidal CdSe and CdTe quantum dots. Even when volume differences are accounted for, band edge NW cross-sections are larger by up to a factor of 8. When considered along with their intrinsic polarization sensitivity, obtained NW cross-sections illustrate fundamental and potentially exploitable differences between 0D and 1D materials.     

水热合成法对纳米氧化锡粉体粒径和形貌的控制研究

利用氯化锡和氨水作为反应试剂,通过水热合成技术制备了近球形,棒状,椭球形,六角形等粉体形貌和粒径范围从4 nm至120 nm的纳米氧化锡粉体,并对水热合成条件对粉体的粒径和形貌的影响进行了研究。所制备的粉体的XRD分析结果显示,合成温度在160 ℃以上并且合成时间在3 h以上,粉体全部具有氧化锡晶体结构。利用透射电镜对粉体的粒径和形貌观察发现,二氧化锡粉体的形貌和粒径可通过改变溶液的浓度,合成温度与合成时间进行有效的控制,其中合成温度与溶液浓度不仅可以控制合成粉体的粒径,而且可以控制粉体的形貌。     

Synthesis of core/shell nanoparticles of Au/CdSe via Au-Cd bialloy precursor

We present a novel method for the preparation of ultrasmall Au/CdSe core/shell particles. Au-Cd bialloy particles of 4.7 nm diameter were prepared as the precursor. The Cd component in the precursor reacted with the Se source at a temperature of 205 degrees C and was heated to 250 degrees C, leading to formation of a Au/CdSe core/shell structure. The sizes of Au/CdSe nanoparticles have a narrow distribution with an average size of 6.0 nm and Au core of 2.2 nm diameter. The X-ray diffraction pattern and the images of the high-resolution electron transmission microscopy show that the Au cores and the CdSe shells of Au/CdSe core/shell nanoparticles are both well crystallized, and the CdSe shells are in a cubic phase. The absorption spectrum of the Au/CdSe nanoparticles combines the absorption behaviors of the Au cores and the CdSe shells.     

Interactions between water-soluble CdSe quantum dots and gold nanoparticles studied by UV-visible absorption spectroscopy.

The interaction of water-soluble CdSe quantum dots (QDs) with gold (Au) nanoparticles was investigated by ultraviolet visible absorption spectroscopy. The results showed that the aggregation of Au nanoparticles was induced by CdSe QDs. The influences of factors such as the size of Au nanoparticles, acidity, buffer concentration and the concentration ratio of the CdSe QDs to Au nanoparticles were each investigated. The comparison of two different particle sizes (16 and 25 nm) of Au nanoparticles that interact with CdSe QDs in the solution showed that the aggregation of small Au nanoparticles (16 nm) is easier than that of big Au nanoparticles (25 nm). At pH 7.0 phosphate buffer solution (0.02 M), the optimal molar ratio of CdSe:Au is about 3100:1 according to calculations.     

Synthesis and aggregation study of tin nanoparticles and colloids obtained by chemical liquid deposition

Tin colloids (Sn-Colls) and nanoparticles were synthesized by a chemical liquid deposition method (CLD). Sn⁰ was evaporated and codeposited with acetone, 2-propanol, and tetrahydrofurane vapors at 77 K to obtain colloidal dispersions. Sn-Coll were characterized by UV spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy, selected area electron diffraction, thermal analysis, infrared spectroscopy [Fourier transform infrared (FTIR)], and light scattering. TEM micrographs of tin nanoparticles (Sn-Nps) revealed a particle size distribution between 2 and 4 nm for the three solvents used in the synthesis. UV studies showed strong absorption bands in the UV region, suggesting that the Sn-Nps obtained by CLD exhibit quantum confinement and typical bands of plasmons corresponded to aggregated particles. Electrophoresis measurement indicated a significant tendency of particle aggregation along time, which was verified by light scattering studies. The diffraction patterns revealed phases corresponding to metallic tin and FTIR studies showed the interaction Sn-solvent in the metal surface by Sn-O bonds, indicating a solvatation of metallic clusters. Thermal analysis revealed a good thermal stability of Sn-Nps. The mechanism of tin nanoparticles formation was also examined.     

Thiol-stabilized CdSe and CdTe nanocrystals in the size quantization regime: Synthesis,optical and structural properties

We report on the recently developed method for the synthesis, optical, and structural properties of CdSe and CdTe nanocrystals. They were formed in aqueous solutions at moderate temperatures by a wet chemical route in the presence of thiol molecules as effective stabilizing agents. The size-selective precipitation technique was applied for the post-preparative nanoparticle fractionation into a series of CdSe and CdTe nanocrystals with extremely narrow size distributions exhibiting mean cluster sizes in the range of 2 to 4 nm. The nature of stabilizing agent (mercaptoalcohols and mercaptoacids) had an important influence on the particle size and determines largely the photoluminescence properties. The nanocrystals were characterized by means of UV-vis absorption and photoluminescence spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy (HRTEM).     

Synthesis of cadmium telluride quantum wires and the similarity of their effective band gaps to those of equidiameter cadmium telluride quantum dots

High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure and grow along the [002] direction (parallel to the c axis). The size dependence of the effective band gaps in the wires is determined from the absorption spectra and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the effective band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire effective band gaps converge. The origin and magnitude of this threshold diameter are discussed.     

CdTe/CdSe核壳量子点的合成及其在指纹显现中的应用

以CdCl2和Te粉为原料,在水相中合成了CdTe量子点核;通过外延生长在CdTe量子点核上包覆一层CdSe量子点,得到具有良好荧光性能的CdTe/CdSe核壳量子点;采用X射线衍射仪、透射电镜、高分辨透射电镜分析了不同反应条件下合成的CdTe/CdSe核壳量子点的晶体结构和微观结构,并对其进行了荧光光谱等测试和指纹显现分析.结果表明,合成的CdTe和CdTe/CdSe量子点粒径在3～5nm之间,粒径分布窄,水分散性良好;可以通过控制反应时间和Te/Se比等得到在500～700nm显示荧光发射峰的CdTe/CdSe核壳量子点.此外,核壳CdTe/CdSe量子点可以有效地和指纹物质结合,可应用于对铝合金油潜指纹的鉴别.     

CdSe包覆层数对水溶性CdTe/CdSe (II型)核壳量子点的光学特性和微观结构的影响

在水相合成的CdTe量子点的体系中通过分批次加入新鲜配制的NaHSe和CdCl2溶液,制备出了CdSe包覆层数不同的CdTe/CdSe核壳量子点,并着重考察了CdSe包覆层数对CdTe/CdSe核壳量子点的光学特性以及微观结构的影响.与CdTe量子点相比,CdSe单层包覆的CdTe/CdSe核壳量子点的吸收峰和荧光发射峰出现明显红移;随着CdSe包覆层数的增多,CdTe/CdSe核壳量子点吸收光谱的覆盖范围向长波方向扩展,荧光发射峰强度逐步下降,荧光寿命大幅延长,体现出Ⅱ型核壳量子点的特征.X射线衍射(XRD)分析表明,随着CdSe包覆层数的增多,CdTe/CdSe核壳量子点的粉末衍射峰由CdTe衍射峰位置逐步向CdSe衍射峰位置靠近.CdTe/CdSe核壳量子点因其延伸到近红外区域的宽吸收特性致使其在太阳电池领域具有重要的应用前景.     

Preparation and characterization of polyvinyl alcohol-capped CdSe nanoparticles at room temperature

Polyvinyl alcohol (PVA)-capped CdSe nanoparticles were successfully prepared by a one-step solution growth technique at room temperature and ambient pressure. X-ray diffraction, transmission electron microscopy, infrared spectra, and X-ray photoelectron spectra were used to characterize the final product. The as-prepared CdSe nanocrystals were well dispersed and uniform in shape and the diameter of the particles was confined within 8 nm. Ultraviolet-visible absorption spectra were used to study the confined growth process of PVA-capped CdSe nanoparticles. Photoluminescence measurement showed the near band-edge luminescence of the final product.     

Synthesis of germanium sols via thermal decomposition of gaseous germane

A new approach to the synthesis of colloidal Ge nanoparticles in high-boiling solvent via thermal decomposition of germane is presented. Obtained products were analyzed by XRD, XRF, TEM, absorption spectrophotometry, Raman IR and PL spectroscopy. Ge nanoparticles have mean sizes about 4–5 nm, are apparently amorphous and nonspherical.     

Highly luminescent CdTe/CdSe colloidal heteronanocrystals with temperature-dependent emission color

In this work we present the preparation of highly luminescent anisotropic CdTe/CdSe colloidal heteronanocrystals. The reaction conditions used (low temperature, slow precursor addition, and surfactant composition) resulted in a tunable shape from prolate to branched CdTe/CdSe nanocrystals. Upon CdSe shell growth the heteronanocrystals show a gradual evolution from type-I to type-II optical behavior. These heteronanocrystals show a remarkably high photoluminescence quantum yield (up to 82%) and negligible thermally induced quenching up to temperatures as high as 373 K.     

Synthesis and optical properties of quasi-2D CdS x Se1 − x nanoparticles

Colloidal 2D CdS _x Se_{1 ? x} nanoparticles have been synthesized by a solution method in octadecene using oleic acid as a stabilizer. Growth of quasi-2D nanoparticles has been promoted by the presence of cadmium acetate in the reaction mixture. The resulting nanoparticles are platelets with lateral sizes 20–30 nm. The absorption and luminescence spectra of these nanoparticles show narrow bands of lh-e and hh-e exciton transitions corresponding to 2D systems. The spectral position of the lowest energy hh-e transition monotonically changes within 382–461 nm with a change in the composition of nanoparticles. The observed absorption bands are broader than those for the individual CdSe and CdS nanoparticles. The suggested method makes it possible to vary the exciton band position for quasi-2D nanoparticles by changing their composition.     

Luminescence 3D-ordered porous materials composed of CdSe and CdTe nanocrystals

The luminescence porous materials of CdTe or CdSe nanocrystals (NCs) were prepared by filling the corresponding NCs into the voids of colloidal crystal by co-deposition of polymer beads and NCs. After removing the beads with tetrahydrofuran (THF), the 3D-ordered porous materials of CdTe (or CdSe) NCs were obtained. The wavelength of maximum photoluminescence of the NCs porous material shows obvious red shift compared with their aqueous dispersion. Under the excitation of high-energy electron the porous materials of CdTe and CdSe NCs will emit photons that can be collected to form a cathode luminescence (CL) image.     

Hexagonal‐Shaped Tin Glycolate Particles: A Preliminary Study of Their Suitability as Li‐Ion Insertion Electrodes

Tin glycolate particles were prepared by a simple, one‐step, polyol‐mediated synthesis in air in which tin oxalate precursor was added to ethylene glycol and heated at reflux. Hexagonal‐shaped, micron‐sized tin glycolate particles were formed when the solution had cooled. A series of tin oxides was produced by calcination of the synthesized tin glycolate at 600–800 °C. It was revealed that the micron‐sized, hexagonal‐shaped tin glycolate now consisted of nanosized tin‐based particles (80–120 nm), encapsulated within a tin glycolate shell. XRD, TGA, and FT‐IR measurements were conducted to account for the three‐dimensional growth of the tin glycolate particles. When applied as an anode material for Li‐ion batteries, the synthesized tin glycolate particles showed good electrochemical reactivity in Li‐ion insertion/deinsertion, retaining a specific capacity of 416 mAh g^?1 beyond 50 cycles. This performance was significantly better than those of all the other tin oxides nanoparticles (<160 mAh g^?1) obtained after heat treatment in air. We strongly believe that the buffering of the volume expansion by the glycolate upon Li–Sn alloying is the main factor for the improved cycling of the electrode.     

Super Sensitization: Grand Charge (Hole/Electron) Separation in ATC Dye Sensitized CdSe,CdSe/ZnS Type‐I,and CdSe/CdTe Type‐II Core–Shell Quantum Dots

Ultrafast charge‐transfer dynamics has been demonstrated in CdSe quantum dots (QD), CdSe/ZnS type‐I core–shell, and CdSe/CdTe type‐II core–shell nanocrystals after sensitizing the QD materials by aurin tricarboxylic acid (ATC), in which CdSe QD and ATC form a charge‐transfer complex. Energy level diagrams suggest that the conduction and valence band of CdSe lies below the LUMO and the HOMO level of ATC, respectively, thus signifying that the photoexcited hole in CdSe can be transferred to ATC and that photoexcited ATC can inject electrons into CdSe QD, which has been confirmed by steady state and time‐resolved luminescence studies and also by femtosecond time‐resolved absorption measurements. The effect of shell materials (for both type‐I and type‐II) on charge‐transfer processes has been demonstrated. Electron injection in all the systems were measured to be <150 fs. However, the hole transfer time varied from 900 fs to 6 ps depending on the type of materials. The hole‐transfer process was found to be most efficient in CdSe QD. On the other hand, it has been found to be facilitated in CdSe/CdTe type‐II and retarded in CdSe/ZnS type‐I core–shell materials. Interestingly, electron injection from photoexcited ATC to both CdSe/CdTe type‐II and CdSe/ZnS type‐I core–shell has been found to be more efficient as compared to pure CdSe QD. Our observation suggests the potential of quantum dot core–shell super sensitizers for developing more efficient quantum dot solar cells.     

Aqueous synthesis of glutathione-capped CdTe/CdS/ZnS and CdTe/CdSe/ZnS core/shell/shell nanocrystal heterostructures

Here we demonstrate the aqueous synthesis of colloidal nanocrystal heterostructures consisting of the CdTe core encapsulated by CdS/ZnS or CdSe/ZnS shells using glutathione (GSH), a tripeptide, as the capping ligand. The inner CdTe/CdS and CdTe/CdSe heterostructures have type-I, quasi-type-II, or type-II band offsets depending on the core size and shell thickness, and the outer CdS/ZnS and CdSe/ZnS structures have type-I band offsets. The emission maxima of the assembled heterostructures were found to be dependent on the CdTe core size, with a wider range of spectral tunability observed for the smaller cores. Because of encapsulation effects, the formation of successive shells resulted in a considerable increase in the photoluminescence quantum yield; however, identifying optimal shell thicknesses was required to achieve the maximum quantum yield. Photoluminescence lifetime measurements revealed that the decrease in the quantum yield of thick-shell nanocrystals was caused by a substantial decrease in the radiative rate constant. By tuning the diameter of the core and the thickness of each shell, a broad range of high quantum yield (up to 45%) nanocrystal heterostructures with emission ranging from visible to NIR wavelengths (500-730 nm) were obtained. This versatile route to engineering the optical properties of nanocrystal heterostructures will provide new opportunities for applications in bioimaging and biolabeling.