A mild phosphine-free synthesis of alkylamine-capped CdSe nanocrystals

A new phosphine-free approach has been developed to synthesize high-quality cadmium selenide (CdSe) nanocrystals with cubic zinc-blende structure, by using the highly reactive selenium (Se) precursor at milder temperature than that used in the traditional phosphine route. This Se precursor was obtained from the reduction of Se powder by sodium borohydride in N,N-dimetbylformamide, in the absence of phosphine. Without the addition of other long-chain coordinating substances in this approach, the alkylamines such as dodecylamine (DDA) and octylamine (OA) were used as reaction solvents, and they also acted as surface capping reagents to produce DDA-capped and OA-capped CdSe NCs, respectively. The rapid nucleation and slow growth were observed by ultraviolet-visible absorption spectrum. The resulting OA-capped CdSe NCs grew faster compared with DDA-capped CdSe NCs under the same other conditions. These as-synthesized CdSe nanocrystals showed relatively narrow size distribution and high photoluminescence quantum efficiency (up to 9.4% for OA-capped CdSe NCs). This mild approach is low cost, relatively low danger and high production yield (approximately 80%), indicating that it is very effective for the phosphine-free synthesis of alkylamine-capped CdSe nanocrystals.     

Low-temperature noninjection approach to homogeneously-alloyed PbSe(x)S(1-x) colloidal nanocrystals for photovoltaic applications

Homogeneously alloyed PbSe(x)S(1-x) nanocrystals (NCs) with their excitonic absorption peaks in wavelength shorter than 1200 nm were developed for photovoltaic (PV) applications. Schottky-type solar cells fabricated with our PbSe?.?S?.? NCs as their active materials reached a high power conversion efficiency (PCE) of 3.44%, with an open circuit voltage (V(oc)) of 0.49 V, short circuit photocurrent (J(sc)) of 13.09 mA/cm2, and fill factor (FF) of 0.54 under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm2. The syntheses of the small-sized colloidal PbSe(x)S(1-x) NCs were carried out at low temperature (60 °C) with long growth periods (such as 45 min) via a one-pot noninjection-based approach in 1-octadecene (ODE), featuring high reaction yield, high product quality, and high synthetic reproducibility. This low-temperature approach employed Pb(oleate)? as a Pb precursor and air-stable low-cost thioacetamide (TAA) as a S source instead of air-sensitive high-cost bis(trimethylsilyl)sulfide ((TMS)?S), with n-tributylphosphine selenide (TBPSe) as a Se precursor instead of n-trioctylphosphine selenide (TOPSe). The reactivity difference of TOPSe made from commercial TOP 90% and TBPSe made from commercial TBP 97% and TBP 99% was addressed with in situ observation of the temporal evolution of NC absorption and with 31P nuclear magnetic resonance (NMR). Furthermore, the addition of a strong reducing/nucleation agent diphenylphosphine (DPP) promoted the reactivity of the Pb precursor through the formation of a Pb-P complex, which is much more reactive than Pb(oleate)?. Thus, the reactivity of TBPSe was increased more than that of TAA. The larger the DPP-to-Pb feed molar ratio, the more the Pb-P complex, the higher the Se amount in the resulting homogeneously alloyed PbSe(x)S(1-x) NCs. Therefore, the use of DPP allowed reactivity match of the Se and S precursors and led to sizable nucleation at low temperature so that long growth periods became feasible. The present study brings insight into the formation mechanism of monomers, nucleation/growth of colloidal composition-tunable NCs, and materials design and synthesis for next-generation low-cost and high-efficiency solar cells.     

Facile synthesis of CdSe nanocrystals for bulk-heterojunction solar cells

Oleic acid (OA)-capped cadmium selenide (CdSe) nanocrystals (NCs) have been synthesized via a new high-scale route. X-ray diffraction and transmission electron microscopy confirmed that rod-like hexagonal (wurtzite) CdSe NCs with an average size of 10 nm were obtained via this new route. The obtained CdSe NCs were treated individually with pyridine and tert-butylamine (t-BA) for ligand exchange. Fourier transform infrared spectra of the as-synthesized and treated CdSe NCs confirmed the removal of OA ligands from the surface of CdSe NCs after treatments with pyridine and t-BA. Bulk-heterojunction (BHJ) solar cell devices were prepared using untreated and surface treated CdSe NCs blended with poly (3-hexylthiophene-2,5-diyl) (P3HT) polymer. BHJ solar cell devices made from P3HT:(surface treated CdSe NCs) blends showed greater improvement in photovoltaic performances compared to P3HT:(untreated CdSe NCs) blend. The improvement in photovoltaic performances was due to the increase of electron mobility in P3HT:(CdSe NCs) blends after surface treatment of CdSe NCs.     

不同温度下硒化镉(CdSe)量子点的生长及荧光性研究

研究了以氧化镉（CdO）和硒（Se）粉为前驱体,在三辛基膦（TOP）和油酸中合成无机半导体量子点（quantum dots, QDs）CdSe.研究了在不同的反应温度下粒子的生长,通过紫外吸收光谱（UV-Vis）、荧光发射光谱（PL）、透射电子显微镜（TEM）等手段跟踪反应过程并对样品性能进行了表征.实验结果表明,反应温度和反应时间对量子点的生长和荧光性能有很大的影响.     

Synthesis of high quality zinc-blende CdSe nanocrystals and their application in hybrid solar cells

Oleic acid (OA) modified zinc-blende cadmium selenium nanocrystals (NCs) with different diameters, 3-5?nm, have been prepared. We find that the morphology and fluorescent properties of the samples are related to the preparation conditions such as the chain-length and concentration of the cadmium precursor as well as the concentration of OA. The hybrid solar cells based on the obtained spherical CdSe NCs as an acceptor and Poly(2-methoxy-5-(2'-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV) as a donor show an energy conversion efficiency (ECE) as high as 0.85%, three times higher than that reported before for spherical CdSe NCs/conjugated polymer hybrid solar cells. When poly(3-hexylthiophene) (P3HT) is used as the donor phase instead of MEH-PPV, the energy conversion efficiency increases up to 1.08%. The solar cell based on CdSe NCs/conjugated polymer has the potential to open up new production technologies for hybrid solar cells based on semiconductor NCs.     

Sequential synthesis of type II colloidal CdTe/CdSe core-shell nanocrystals

Colloidal type II CdTe/CdSe nanocrystals were synthesized by sequential addition of a tri-n-octylphosphine telluride (TOPTe)/TOP solution and several shell-precursor solutions to a CdO/TOP solution; the shell-precursor solutions consisted of CdO and TOPSe in TOP. For the growth of the CdTe core, the TOPTe/TOP solution was swiftly added to the CdO/TOP solution at a higher temperature (300 degrees C) than the growth temperature (250 degrees C). For the growth of the CdSe shell, in contrast, the CdO/TOPSe/TOP solution was slowly added to the CdTe/TOP solution at a lower temperature than the growth temperature (200-240 degrees C). The temporal evolution of the optical properties of the growing core-shell nanocrystals was monitored in detail. During the growth of the CdSe shell, the core-shell nanocrystals exhibited interesting changes in photoluminescence (PL) properties. The highest PL efficiency (approximately 38 %) was detected from core-shell nanocrystals with a CdSe shell thickness of 0.4-0.5 nm (indicated by TEM); the formation of the first monolayer is proposed. Our synthetic approach is well suited to a practical realization of engineering materials with bandgaps in the near-IR and IR spectral ranges.     

In-situ observation of nucleation and growth of PbSe magic-sized nanoclusters and regular nanocrystals

In-situ observation of the temporal evolution of the absorption of PbSe nanocrystals (NCs) via a low-temperature noninjection approach is presented. Based on a model reaction of lead oleate (Pb(OA)(2) ) and n-trioctylphosphine selenide (TOPSe) in 1-octadecene at 35-80 °C, the use of commercially available TOP (90 or 97%) in affecting the formation of the NCs is explored. TOPSe solutions made from TOP 90% exhibited higher reactivity than those made from TOP 97%. (31)P NMR spectroscopy detected no dioctylphosphine selenide (DOPSe) but some DOP in ≈1.0 M TOPSe/TOP solution (made from TOP 90%), as well as no diphenylphosphine selenide (DPPSe) when DPP was added to the ≈1.0 M solution. Hence, it is proposed that, for the formation of PbSe monomers, an indirect pathway dominates with the formation of a Pb-P complex/intermediate, which results from the activation of Pb(OA)(2) by a phosphine compound (such as DPP, DOP, or TOP) and in turn reacts with TOPSe. With the use of TOP 90% and the addition of secondary phosphine DPP, the formation of PbSe magic-sized nanoclusters (MSNCs) and regular NCs (RNCs) is investigated. With proper tuning of the synthesis conditions, the formation of various PbSe MSNCs versus RNCs is monitored in situ with versus without the addition of DPP, or at different reaction temperatures but otherwise identical synthetic formulation and reaction parameters. Accordingly, the degree of supersaturation (DS) of the PbSe monomer affecting the development of these PbSe MSNCs versus RNCs is proposed; the higher the DS, the more the MSNCs are favored. Also, surface-determined cluster-cluster aggregation is proposed to be the growth mechanism for both the RNCs and MSNCs. For the former, quantized growth is followed by continuous growth. For the latter, the sizes of the magic-sized families are calculated.     

Colloidal CdSe nanocrystals from tri-n-octylphosphine: Part II: control of growth rate for high quality and large-scale production by tuning Cd-to-Se stoichiometry

Colloidal CdSe nanocrystals were synthesized in reaction media consisting of tri-n-octylphosphine (TOP) without addition of other species; the single-step approach used cadmium oxide (CdO) and TOPSe as Cd and Se sources, respectively. The temporal evolution of the optical properties of the growing TOP-capped CdSe nanocrystals was monitored for a couple of hours, showing that there are two distinguishable stages of growth: an early stage (less than 5 minutes) and a later stage; the growth kinetics of the two stages is a function of the Cd-to-Se precursor molar ratios. A rational choice of 2-6Cd-to-1Se molar ratio was found, based on the temporal evolution of the photoluminescent (PL) efficiency (studied as PL intensity and sensitivity to the media of dispersion, and non-resonant Stokes shifts). For a 2Cd-to-1Se synthesis, the growth in size was slow in the early stages and became fast in the later stages; this fast-later-stage feature could be suppressed by going to a synthesis with a 4-6Cd-to-1Se mole ratio: the nanocrystals between 0.5-60 min growth time exhibit very much similar optical properties, with less than 19 nm redshift of bandgap absorption and emission occurring. Thus, the synthetic route developed here, with a rational 4-6Cd-to-1Se molar ratio, enables us to produce high-quality CdSe nanocrystals on a large-scale with a high degree of synthetic reproducibility. The insights gained facilitate a deeper understanding of the concept of what constitutes high-quality nano-crystals: high PL efficiency resulting from a low growth rate, which can be thoroughly and readily investigated by the red-shift rate of the band-gap peak positions; in addition, the insights gained help us to define a proper synthetic approach for large-scale production with high-quality product.     

Study on the photoluminescence character of water-soluble CdSe nanocrystals under ambient conditions

Water soluble, thioglycolic acid (TGA) modified CdSe nanocrystals (NCs) have been prepared in aqueous media by the reaction between Cd2+ and NaHSe. Although initially these quantum dots (QDs) display photoluminescence (PL) with very low quantum yields (QY), upon prolonged exposure to ambient light, a strong PL enhancement by illumination is observed which leads to water soluble QDs with high luminescence. This result may have important application potential in biological or other fields. The primary reason for the luminescence enhancement is concluded to be the incorporation of sulfide ions from TGA into the lattice of CdSe NCs and the subsequent formation of alloy structures. Moreover, the CdSe/CdS core-shell structured QDs synthesized in aqueous solutions also consolidate this conclusion.     

Layer-by-layer growth of CdSe-based nanocrystal light-emitting diodes

The partial exchange of surface-passivating trioctylphosphine oxide (TOPO) on CdSe and ZnS-clad CdSe (CdSe/ZnS) nanocrystals with primary amines was utilized to grow ultra-thin films of these nanocrystals under nonaqueous conditions. This growth was achieved using 1,12-diaminododecane in a layer-by-layer assembly format, where one of the amino groups binds with the nanocrystal surface and the other regenerates the interface for further binding of nanocrystals. The nature of the growth is dependent on the relative surface affinity between the TOPO and the primary amine toward the zinc or cadmium sites on the nanocrystals. Using this technique, high-quality luminescent films of these nanocrystals can be built with well-defined thicknesses. Electroluminescent devices have been fabricated using this methodology.     

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.     

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．     

Rapid synthesis of CdSe nanocrystals in aqueous solution at room temperature

Water-soluble thioglycolic acid-capped CdSe nanocrystals (NCs) were prepared in aqueous solution at room temperature. We investigated the effects of pH values on the fluorescence intensity of the as-prepared CdSe NCs, and discussed the influence of the initial pH values on the fluorescence property. Their mean diameter was estimated to be 1.9 nm depending on the initial pH values in the preparation, the photoluminescence quantum yield could reach as high as 1.9%, almost comparable to the CdSe NCs prepared by an organometallic route. Finally, the products were characterized by Fourier transform infrared spectrometry (FTIR), atomic force microscope (AFM) and X-ray powder diffraction (XRD). AFM image showed that the NCs were ball-shaped with good dispersibility. XRD analysis disclosed that the CdSe NCs were of cubic zinc-blended structure.     

Sr0.95Zn0.05Se:Eu2+ and CdSe/ZnS nanocrystals hybrid phosphors for enhancing color rendering index of white light emitting diode

In this study, the yellow emitting cubic structure of Sr0.95Zn0.05Se:Eu2+ phosphors were prepared by high temperature solid state reaction. The Sr0.95Zn0.05Se:Eu2+ phosphors exhibited strong excitation intensity under 400-460 nm region, and broad band emission appeared at around 545-600 nm due to the d-f transition of Eu2+. To enhance the red emission, HDA/TOP/TOPO capped CdSe/ZnS NCs were synthesized via fast nucleation and slow growth method. The narrow emission peak was located at 615 nm with 69% of high quantum yield. Bright white emission was generated by combining a 460 nm InGaN LED chip with CdSe/ZnS NCs and Sr0.95Zn0.05Se:Eu2+ hybrid phosphors. The fabricated white LEDs showed warm white light with acceptable CIE chromaticity coordinate variation from (0.343, 0.255) at 20 mA to (0.335, 0.250) at 50 mA. The addition of CdSe/ZnS NCs contributed to the extension of white light spectrum by supplement of the red region. The color rendering index was largely enhanced from 41.7 to 79.7 compared to the Sr0.95Zn0.05Se:Eu2+ based phosphors white LED.     

Aqueous synthesis of luminescent magic sized CdSe nanoclusters

Highly stable water-soluble CdSe nanoclusters (NCs) with magic size were successfully synthesized using homocysteine (HCY) as capping ligands. Their sizes were tunable between 1.2 and 2.0 nm depending on reflux time. The final products were characterized by UV-vis absorption, steady and time-resolved photoluminescence (PL) spectra, X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analysis showed that the HCY-capped CdSe NCs were of the cubic structure, UV-vis absorption spectra and HRTEM micrograph exhibited that the NCs were nearly monodisperse and relatively uniform. The as-prepared CdSe NCs had a PL quantum yield of up to 1.4%, almost comparable to the CdSe magic sized clusters prepared by an organometallic route.     

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.     

嵌段聚合物模板合成CdSe纳米粒子及表征

以羟基化SBS(SBS-OH)为模板,N,N-二甲基甲酰胺(DMF)为溶剂,硒代硫酸钠及乙酸镉为前驱物,"原位反应"制备了CdSe纳米粒子.通过紫外-可见吸收光谱、荧光光谱(PL)和透射电子显微镜(TEM)等方法对CdSe纳米粒子的光学性质及形貌进行了表征.结果表明,利用SBS-0H的两亲性质,可以在DMF中得到具有明...     

Synthesis and size dependent optical studies in CdSe quantum dots via inverse micelle technique

Cadmium selenide quantum dots (CdSe QDs) were successfully synthesized without using trioctylphosphine (TOP). The XRD pattern showed zinc-blend phase of the CdSe QDs. The absorption and PL spectra exhibit a strong blue shift as the QDs size decreases due to the quantum confinement effect. In addition, the quantum efficiency of CdSe QDs with TOP capping is higher than CdSe QDs with oleic acid capping. TEM image shows a spherical shape, compact and dense structure of CdSe QDs. A good agreement between the Tauc's model and experimentally measured absorption spectra of CdSe QDs is achieved. The FTIR peak at ～1712 cm^?1 spectra confirms the influence of oleic acid as a capping agent.     

微反应器中利用非配位有机溶剂法可控合成CdSe量子点

采用微反应器技术, 在低毒性的非配位有机溶剂十八烯(ODE)/配体十八烯酸(OA)反应体系中对CdSe量子点进行了尺寸可控合成. 详细探讨了制备条件对生成的CdSe量子点的粒子数及粒子尺寸的影响. 结果表明, 反应温度对生成的粒子数不产生显著影响, 但相对较高的温度有利于具有较强荧光性的CdSe量子点生成. 在固定其他反应条件下, 过量的Se有利于生成大量的小粒子, 当n(Cd):n(Se)=1:5时, 产率可接近100%. 反应物中配体的量直接影响到单体的浓度, 通过控制配体OA浓度和反应时间, 成功地制备了一系列显示从蓝色到红色荧光, 粒径在2.2~4.3nm的CdSe量子点.