Developing a theoretical relationship between electrical resistivity, temperature, and film thickness for conductors

We address two aspects of general interest for the chemical synthesis of colloidal semiconductor nanocrystals: (1) the rational design of the synthesis protocol aiming at the optimization of the reaction parameters in a minimum number of experiments; (2) the transfer of the procedure to the gram scale, while maintaining a low size distribution and maximizing the reaction yield. Concerning the first point, the design-of-experiment (DOE) method has been applied to the synthesis of colloidal CdSe nanocrystals. We demonstrate that 16 experiments, analyzed by means of a Taguchi L₁₆ table, are sufficient to optimize the reaction parameters for controlling the mean size of the nanocrystals in a large range while keeping the size distribution narrow (5-10%). The DOE method strongly reduces the number of experiments necessary for the optimization as compared to trial-and-error approaches. Furthermore, the Taguchi table analysis reveals the degree of influence of each reaction parameter investigated (e.g., the nature and concentration of reagents, the solvent, the reaction temperature) and indicates the interactions between them. On the basis of these results, the synthesis has been scaled up by a factor of 20. Using a 2-L batch reactor combined with a high-throughput peristaltic pump, different-sized samples of CdSe nanocrystals with yields of 2-3 g per synthesis have been produced without sacrificing the narrow size distribution. In a similar setup, the gram-scale synthesis of CdSe/CdS/ZnS core/shell/shell nanocrystals exhibiting a fluorescence quantum yield of 81% and excellent resistance of the photoluminescence in presence of a fluorescent quencher (aromatic thiol) has been achieved. PACS: 81.20.Ka, 81.07.Bc, 78.67.Bf     

Microwave Synthesis of Nearly Monodisperse Core/Multishell Quantum Dots with Cell Imaging Applications

We report in this article the microwave synthesis of relatively monodisperse, highly crystalline CdSe quantum dots (QDs) overcoated with Cd_0.5Zn_0.5S/ZnS multishells. The as-prepared QDs exhibited narrow photoluminescence bandwidth as the consequence of homogeneous size distribution and uniform crystallinity, which was confirmed by transmission electron microscopy. A high photoluminescence quantum yield up to 80% was measured for the core/multishell nanocrystals. Finally, the resulting CdSe/Cd_0.5Zn_0.5S/ZnS core/multishell QDs have been successfully applied to the labeling and imaging of breast cancer cells (SK-BR3).     

Optimization of Parameters for Synthesis of MFI Nanoparticles by Taguchi Robust Design

MFI‐type zeolite was successfully synthesized by hydrothermal crystallization of clear synthesis mixtures. A statistical experimental design method (the Taguchi method with an L8 orthogonal array) was implemented to optimize the experimental conditions for the preparation of MFI nanocrystals with respect to particle size and distribution as the desirable properties. In the Taguchi experimental design, crystallization temperature, water content, template/silica molar ratio, aluminum content, as well as the presence of alkaline cations were chosen as significant parameters affecting the properties. It was shown that water and aluminum content of the synthesis solution were the most important parameters affecting particle size and distribution. The MFI nanocrystals with an average particle size of 95 nm and the narrow particle size distribution of ± 8.5 nm were synthesized under optimum conditions.     

Formation of PbSe/CdSe Core/Shell Nanocrystals for Stable Near-Infrared High Photoluminescence Emission

PbSe/CdSe core/shell nanocrystals with quantum yield of 70% were obtained by the “successive ion layer adsorption and reaction” technology in solution. The thickness of the CdSe shell was exactly controlled. A series of spectral red shifts with the CdSe shell growth were observed, which was attributed to the combined effect of the surface polarization and the expansion of carriers’ wavefunctions. The stability of PbSe nanocrystals was tremendously improved with CdSe shells.     

Colloidal chemical synthesis and formation kinetics of uniformly sized nanocrystals of metals, oxides, and chalcogenides

Nanocrystals exhibit interesting electrical, optical, magnetic, and chemical properties not achieved by their bulk counterparts. Consequently, to fully exploit the potential of nanocrystals, the synthesis of nanocrystals must focus on producing materials with uniform size and shape. Top-down physical processes can produce large quantities of nanocrystals, but controlling the size is difficult with these methods. On the other hand, colloidal chemical synthetic methods can produce uniform nanocrystals with a controlled particle size. In this Account, we present our synthesis of uniform nanocrystals of various shapes and materials, and we discuss the kinetics of nanocrystal formation. We employed four different synthetic approaches including thermal decomposition, nonhydrolytic sol-gel reactions, thermal reduction, and use of reactive chalcogen reagents. We synthesized uniform oxide nanocrystals via heat-up methods. This method involved slowly heat-up reaction mixtures composed of metal precursors, surfactants, and solvents from room temperature to high temperature. We then held reaction mixtures at an aging temperature for a few minutes to a few hours. Kinetics studies revealed a three-step mechanism for the synthesis of nanocrystals through the heat-up method with size distribution control. First, as metal precursors thermally decompose, monomers accumulate. At the aging temperature, burst nucleation occurs rapidly; at the end of this second phase, nucleation stops, but continued diffusion-controlled growth leads to size focusing to produce uniform nanocrystals. We used nonhydrolytic sol-gel reactions to synthesize various transition metal oxide nanocrystals. We employed ester elimination reactions for the synthesis of ZnO and TiO(2) nanocrystals. Uniform Pd nanoparticles were synthesized via a thermal reduction reaction induced by heating up a mixture of Pd(acac)(2), tri-n-octylphosphine, and oleylamine to the aging temperature. Similarly, we synthesized nanoparticles of copper and nickel using metal(II) acetylacetonates. Ni/Pd core/shell nanoparticles were synthesized by simply heating the reaction mixture composed of acetylacetonates of nickel and palladium. Using alternative chalcogen reagents, we synthesized uniform nanocrystals of various metal chalcogenides. Uniform nanocrystals of PbS, ZnS, CdS, and MnS were obtained by heating reaction mixtures composed of metal chlorides and sulfur dissolved in oleylamine. In the future, a detailed understanding of nanocrystal formation kinetics and synthetic chemistry will lead to the synthesis of uniform nanocrystals with controlled size, shape, and composition. In particular, the synthesis of uniform nanocrystals of doped materials, core/shell materials, and multicomponent materials is still a challenge. We expect that these uniformly sized nanocrystals will find important applications in areas including information technology, biomedicine, and energy/environmental technology.     

Synthesis of Monodisperse Nanocrystals via Microreaction: Open-to-Air Synthesis with Oleylamine as a Coligand

Microreaction provides a controllable tool to synthesize CdSe nanocrystals (NCs) in an accelerated fashion. However, the surface traps created during the fast growth usually result in low photoluminescence (PL) efficiency for the formed products. Herein, the reproducible synthesis of highly luminescent CdSe NCs directly in open air was reported, with a microreactor as the controllable reaction tool. Spectra investigation elucidated that applying OLA both in Se and Cd stock solutions could advantageously promote the diffusion between the two precursors, resulting in narrow full-width-at-half maximum (FWHM) of PL (26 nm). Meanwhile, the addition of OLA in the source solution was demonstrated helpful to improve the reactivity of Cd monomer. In this case, the focus of size distribution was accomplished during the early reaction stage. Furthermore, if the volume percentage (vol.%) of OLA in the precursors exceeded a threshold of 37.5%, the resulted CdSe NCs demonstrated long-term fixing of size distribution up to 300 s. The observed phenomena facilitated the preparation of a size series of monodisperse CdSe NCs merely by the variation of residence time. With the volume percentage of OLA as 37.5% in the source solution, a 78 nm tuning of PL spectra (from 507 to 585) was obtained through the variation of residence time from 2 s to 160 s, while maintaining narrow FMWH of PL (26–31 nm) and high QY of PL (35–55%).     

Inkjet Printing of Aqueous Photoluminescent CdSe/CdS Nanorods on Solid Substrates

In order to deposit colloidal nanocrystals on different substrates, a large variety of methodologies have been developed, such as dipcoating, spin coating, or spray coating. The deposition of CdSe/CdS core‐shell nanorods (NRs) with a conventional office printer onto conducting tin‐doped indium oxide glass surfaces is presented. This involves the preparation of water‐based inks containing the NRs. To improve the connection between the CdSe/CdS NRs and the underlying substrates, the glass surfaces were modified by means of silanes. Homogeneous films were obtained by consecutively printing several layers of CdSe/CdS NRs.     

Mesoporous carbon confined conversion of silica nanoparticles into zeolite nanocrystals

A novel synthesis method has been developed to synthesize zeolite nanocrystals with controllable size and size distribution through conversion of colloidal silica nanoparticles in a mesoporous carbon matrix. Colloidal silica nanoparticles serving as the silica source as well as the hard template were embedded in the mesoporous carbon via in situ polymerization of furfuryl alcohol in the presence of tri-block copolymer (Pluronic P123), and then reacted with an alkaline aqueous solution (Na₂O–Al₂O₃–H₂O) infiltrated through the mesoporous channels of the carbon matrix. The synthesis of zeolite NaA nanocrystals is demonstrated. SEM images reveal that the sizes of the zeolite NaA nanocrystals obtained are similar to those of the starting silica nanoparticles. As a result, this synthesis method shows the potential for controlling the nanocrystal size and size distribution of silica-containing zeolites by choosing colloidal silica nanoparticles with a desired particle size distribution.     

水热法制备细胞标记用CdSe半导体量子点材料

介绍了水热合成CdSe半导体纳米晶的工艺,调节反应温度及时间有助于加速CdSe纳米晶的生长,使其荧光产率得到显著提高;在一定的初始条件下,随着纳米晶核长大,产物的荧光产率单调增至极值然后逐渐下降;CdSe荧光产率的最大值与反应温度关系密切。实验利用1-乙基–1-3-(3-二甲基氨基-丙基)–碳化二亚胺将所制备的CdSe与转铁蛋白偶联成生物探针,成功标记了人肺腺癌细胞。     

Facile Phosphine-Free Synthesis of CdSe/ZnS Core/Shell Nanocrystals Without Precursor Injection

A new simple method for synthesis of core/shell CdSe/ZnS nanocrystals (NCs) is present. By adapting the use of cadmium stearate, oleylamine, and paraffin liquid to a non-injection synthesis and by applying a subsequent ZnS shelling procedure to CdSe NCs cores using Zinc acetate dihydrate and sulfur powder, luminescent CdSe/ZnS NCs with quantum yields of up to 36% (FWHM 42–43 nm) were obtained. A seeding-growth technique was first applied to the controlled synthesis of ZnS shell. This method has several attractive features, such as the usage of low-cost, green, and environmentally friendlier reagents and elimination of the need for air-sensitive, toxic, and expensive phosphines solvent. Furthermore, due to one-pot synthetic route for CdSe/ZnS NCs, the approach presented herein is accessible to a mass production of these NCs.     

CdSe@ZnSe@SiO2核壳壳型纳米粒子的制备与表征

通过TOP-TOPO-HDA路线制备了CdSe纳米晶体,并在此基础上通过一步法制备出CdSe@ZnSe核壳纳米粒子。利用环己烷-壬基酚聚（5）氧乙烯醚（NP-5）-水的微乳体系为模板,通过硅烷偶联剂的作用制备出CdSe@ZnSe@SiO2复合纳米粒子,并通过TEM、EDX和UV-Vis等手段对所得纳米粒子进行了表征。     

Specific features of the growth of CdSe nanocrystals in fluorophosphate glasses

The choice of the composition of the fluorophosphate vitreous matrix used for optimizing the growth of CdSe nanocrystals with a narrow size distribution is justified. Investigation of the X-ray diffraction parameters has revealed that nanocrystals with sizes varying over a wide range have a hexagonal structure. The evolution of the absorption spectra of fluorophosphate glasses containing CdSe semiconductor nanocrystals is studied. It is shown that the growth mechanism of CdSe nanocrystals differs radically from the nucleation mechanism. The time dependences of the nanocrystal size exhibit an intricate three-stage behavior. The first stage is accompanied by a shift in the absorption spectrum toward the long-wavelength range. After the first stage, the time dependences are characterized by “fast” and “slow” portions. In the fast portion, the exponent of the time dependence of the nanocrystal size increases from 0.4 to 1.0 with an increase in the annealing temperature. The assumption is made that the spinodal decomposition of phases under hyperbolic diffusion occurs in the system under investigation.     

Synthesis,characterization, and magnetic properties of α-MnS nanocrystals

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl₂ and S[Si(CH₃)₃]₂ in trioctylphosphineoxide. The nanocrystals were characterized using X-ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X-ray diffraction shows that the nanocrystals are of the thermodynamically stable α-MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80-, 40-, and 20-nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, T_N = 130 K, of bulk α-MnS. Below T_N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of T_N with reduced particle size, indicating size dependent magnetic ordering.     

One-Pot Synthesis of Biocompatible CdSe/CdS Quantum Dots and Their Applications as Fluorescent Biological Labels

We developed a novel one-pot polyol approach for the synthesis of biocompatible CdSe quantum dots (QDs) using poly(acrylic acid) (PAA) as a capping ligand at 240°C. The morphological and structural characterization confirmed the formation of biocompatible and monodisperse CdSe QDs with several nanometers in size. The encapsulation of CdS thin layers on the surface of CdSe QDs (CdSe/CdS core–shell QDs) was used for passivating the defect emission (650 nm) and enhancing the fluorescent quantum yields up to 30% of band-to-band emission (530–600 nm). Moreover, the PL emission peak of CdSe/CdS core–shell QDs could be tuned from 530 to 600 nm by the size of CdSe core. The as-prepared CdSe/CdS core–shell QDs with small size, well water solubility, good monodispersity, and bright PL emission showed high performance as fluorescent cell labels in vitro. The viability of QDs-labeled 293T cells was evaluated using a 3-(4,5-dimethylthiazol)-2-diphenyltertrazolium bromide (MTT) assay. The results showed the satisfactory (>80%) biocompatibility of as-synthesized PAA-capped QDs at the Cd concentration of 15 μg/ml.     

Facile synthesis of monodisperse ZnS capped CdS nanocrystals exhibiting efficient blue emission

A new method for the capping of colloidal CdS nanocrystals with ZnS shells is presented. A combination of the monomolecular precursor zinc ethylxanthate (Zn(ex)₂) and zinc stearate was used to replace hazardous organometallic reagents usually applied in this procedure, i.e. bis(trimethylsilyl) sulfide and diethylzinc. Its simple preparation, air-stability and low decomposition temperature of 150 °C make Zn(ex)₂ a very suitable source for the ZnS shell growth. With this precursor, highly luminescent CdS/ZnS core/shell nanocrystals (Q.Y. 35–45%), exhibiting narrow emission linewidths of 15–18 nm (FWHM) in the blue spectral region, can reproducibly be obtained.     

Low temperature solution-phase growth of ZnSe and ZnSe/CdSe core/shell nanowires

High quality ZnSe nanowires (NWs) and complementary ZnSe/CdSe core/shell species have been synthesized using a recently developed solution-liquid-solid (SLS) growth technique. In particular, bismuth salts as opposed to pre-synthesized Bi or Au/Bi nanoparticles have been used to grow NWs at low temperatures in solution. Resulting wires are characterized using transmission electron microscopy and possess mean ensemble diameters between 15 and 28 nm with accompanying lengths ranging from 4-10 μm. Subsequent solution-based overcoating chemistry results in ZnSe wires covered with CdSe nanocrystals. By varying the shell's growth time, different thicknesses can be obtained and range from 8 to 21 nm. More interestingly, the mean constituent CdSe nanocrystal diameter can be varied and results in size-dependent shell emission spectra.     

Synthesis of Efficiently Green Luminescent CdSe/ZnS Nanocrystals Via Microfluidic Reaction

Quantum dots with emission in the spectral region from 525 to 535 nm are of special interest for their application in green LEDs and white-light generation, where CdSe/ZnS core-shell structured nanocrystals (NCs) are among promising candidates. In this study, triple-ligand system (trioctylphosphine oxide–oleic acid–oleylamine) was designed to improve the stability of CdSe NCs during the early reaction stage. With the precisely controlled reaction temperature (285 °C) and residence time (10 s) by the recently introduced microfluidic reaction technology, green luminescent CdSe NCs (λ = 522 nm) exhibiting narrow FWHM of PL (30 nm) was reproducibly obtained. After that, CdSe/ZnS core-shell NCs were achieved with efficient luminescence in the pure green spectral region, which demonstrated high PL QY up to 70% and narrow PL FWHM as 30 nm. The strengthened mass and heat transfer in the microchannel allowed the formation of highly luminescent CdSe/ZnS NCs under low reaction temperature and short residence time (T = 120 °C, t = 10 s). The successful formation of ZnS layer was evidence of the substantial improvement of PL intensity, being further confirmed by XRD, HRTEM, and EDS study.     

Infrared colloidal lead chalcogenide nanocrystals: synthesis, properties, and photovoltaic applications

Simple solution phase, catalyst-free synthetic approaches that offer monodispersed, well passivated, and non-aggregated colloidal semiconductor nanocrystals have presented many research opportunities not only for fundamental science but also for technological applications. The ability to tune the electrical and optical properties of semiconductor nanocrystals by manipulating the size and shape of the crystals during the colloidal synthesis provides potential benefits to a variety of applications including photovoltaic devices, light-emitting diodes, field effect transistors, biological imaging/labeling, and more. Recent advances in the synthesis and characterization of colloidal lead chalcogenide nanocrystals and the achievements in colloidal PbS or PbSe nanocrystals solar cells have demonstrated the promising application of infrared-emitting colloidal lead chalcogenide nanocrystals in photovoltaic devices. Here, we review recent progress in the synthesis and optical properties of colloidal lead chalcogenide nanocrystals. We focus in particular upon the size- and shape-controlled synthesis of PbS, PbSe, and PbTe nanocrystals by using different precursors and various stabilizing surfactants for the growth of the colloidal nanocrystals. We also summarize recent advancements in the field of colloidal nanocrystals solar cells based on colloidal PbS and PbSe nanocrystals.     

CdS/CdSe核壳结构半导体纳米晶结构及光谱特性

采用共沉淀法在水相中将CdS外延生长于CdSe半导体纳米晶(CdSe NCs)表面,制备了结晶形态较好的CdSc/CdS核壳结构半导体纳米晶(CdSe/CdS C/S NCs),并改善了CdSe NCs的荧光性能.通过X射线衍射,透射电镜和选区电子衍射分析证明了核壳结构的形成,并通过紫外可见光吸收光谱和荧光光谱分析证明了核壳结构对CdSe NCs荧光性能的改善.此外,实验结果表明:控制CdSe核合成中Cd前驱体溶液pH值能获得粒径分布较窄的CdSe/CdS C/S NCs;采用不同浓度比的Cd和Se前驱体溶液可以有效调节核壳结构半导体纳米晶的粒径;选择合适的CdSe与CdS摩尔比及壳层中Cd与S摩尔比能改善CdSe/CdS C/S NCs的荧光性能.     

The kinetics of the formation of CdSe nanocrystals in sodium-zinc-silica glass

The kinetics of precipitation of CdSe nanocrystals in sodium-zinc-silica glass subjected to a two-stage heat treatment at different temperatures of the primary and secondary heat treatments has been studied by small-angle X-ray scattering (SAXS). The primary low-temperature heat treatment is accompanied by the nucleation of crystallization centers, i.e., supercritical nucleation centers. During the secondary heat treatment, crystals largely grow on the formed centers. In this case, the crystals are distributed in the bulk of the glass in a certain order, which apparently results from the regular pattern of distribution of the crystallization centers formed during the primary heat treatment. It has been found that the precipitated nanocrystals have a narrow size distribution at all stages of heat treatment. After the completion of precipitation of the crystalline phase, a stable structure is formed. The crystal sizes and the volume fraction of the phase do not change within the measurement accuracy (±1–2%) for the heating times that are at least an order of magnitude longer than the duration of the precipitation stage.