油胺/油酸稳定的CdSe量子点的绿色合成

以液体石蜡为高温反应溶剂,油酸和油胺为混合稳定剂,利用高温热解法一步合成了高质量的CdSe量子点。通过紫外-可见吸收光谱、荧光发射光谱、红外光谱和X射线衍射等手段对量子点的光学性质和结构进行了表征。结果表明,油胺/油酸混合表面活性剂稳定的量子点吸收光谱峰形更尖锐,荧光发射光谱半峰宽更窄。反应温度和反应时间均对量子点的生长过程和光学性质有明显影响,220℃下反应15 min,荧光量子产率可达26%。得到的CdSe量子点为立方晶型,表面同时包覆了油酸和油胺,具有良好的光稳定性。该方法无需使用三烷基膦,价廉环保,且合成的CdSe量子点性质稳定、性能优越,有利于其在分析检测领域中的应用。     

Strong thermal optical nonlinearity caused by CdSe nanoparticles synthesised in smectic ionic liquid crystal

Spectral and nonlinear optical properties of cadmium octanoate composites containing CdSe nanoparticles (NPs) have been studied by using optical absorption spectroscopy and laser scanning technique (Z-scan). CdSe NPs are chemically synthesised in thermotropic ionic liquid crystal (ILC) phase of cadmium octanoate which is used as nanoreactor. Anisotropic glassy nanocomposites are obtained by the rapid cooling of the ILC phase of nanocomposites to room temperature. The sizes of the CdSe NPs are determined from the absorption spectra. The thermo-optical nonlinearity of the new nanocomposites is characterised by extremely large value of the nonlinear refractive index, n₂, under relatively low-powered CW laser irradiation. This nonlinearity is caused by (1) the efficient light-induced heating due to the CdSe NPs strong exciton absorption, and (2) consequent thermal dissipation, which in turn, produces the photoelastic tensions in the glassy smectic matrix.     

表面配体对CdSe量子点发光性质的影响

本文采用热注入法合成了以油胺/油酸为表面配体的、粒径均一的CdSe量子点(CdSe QDs)。调节表面配体交换中辛硫醇与CdSe QDs的比例,研究了表面配体对CdSe QDs光致发光及电致化学发光性质的影响,并提出了CdSe QDs的发光模型。结果表明,辛硫醇表面配体显著影响CdSe QDs的带边发射和深能级陷阱发射,因而导致CdSe QDs光致发光强度的显著降低,以及电致化学发光强度的增加。上述结果为进一步提高量子点的发光性能提供了依据。     

CdSe Nanoplatelets: Living Polymers

Colloidal CdSe nanoplatelets are considered to be excellent candidates for many applications in nanotechnology. One of the current challenges is to self‐assemble these colloidal quantum wells into large ordered structures to control their collective optical properties. We describe a simple and robust procedure to achieve controlled face‐to‐face self‐assembly of CdSe nanoplatelets into micron‐long polymer‐like threads made of up to ～1000 particles. These structures are formed by addition of oleic acid to a stable colloidal dispersion of platelets, followed by slow drying and re‐dispersion. We could control the average length of the CdSe nanoplatelet threads by varying the amount of added oleic acid. These 1‐dimensional structures are flexible and feature a “living polymer” character because threads of a given length can be further grown through the addition of supplementary nanoplatelets at their reactive ends.     

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.     

Facet‐Dependent On‐Surface Reactions in the Growth of CdSe Nanoplatelets

Facet‐dependent on‐surface reactions are systematically studied on zinc‐blende CdSe nanoplatelets with atomically‐flat {001} basal facets and small yet non‐polar side facets. The on‐surface half‐reactions between the surface Se sites and Cd carboxylates in the solution are qualitatively equivalent to those on the spheroidal counterparts. Conversely, the on‐surface half‐reactions between the surface Cd sites and the activated Se precursors in solution show a strong facet‐dependence, which includes three distinguishable stages. In the first stage, the Se precursors adsorb onto the small and non‐polar side facets of the nanoplatelets. The second stage is initiated by the adsorbed Se precursors at the side‐basal plane edges and proceeds from the edges to the center of the basal planes in quasi‐zeroth‐order kinetics. In the third stage, the nanoplatelets are dismantled, which includes the creation of a hole in the middle and a build‐up of thick edges.     

Understanding mercapto ligand exchange on the surface of FePt nanoparticles

Tailoring the surface of nanoparticles is essential for biological applications of magnetic nanoparticles. FePt nanoparticles are interesting candidates owing to their high magnetic moment. Established procedures to make FePt nanoparticles use oleic acid and oleylamine as the surfactants, which make them dispersed in nonpolar solvents such as hexane. As a model study to demonstrate the modification of the surface chemistry, stable aqueous dispersions of FePt nanoparticles were synthesized after ligand exchange with mercaptoalkanoic acids. This report focuses on understanding the surface chemistry of FePt upon ligand exchange with mercapto compounds by conducting X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies. It was found that the mercapto end displaces oleylamine on the Pt atoms and the carboxylic acid end displaces the oleic acid on the Fe atoms, thus exposing carboxylate and thiolate groups on the surface that provide the necessary electrostatic repulsion to form stable aqueous dispersions of FePt nanoparticles.     

CdSe纳米晶/共轭聚合物太阳电池的制备与性能研究

采用有机金属液相法制备了平均粒径为5 nm的CdSe纳米微球(ns-CdSe), 并将其与共轭聚合物(MEH-PPV或P3HT)共混制备了太阳电池器件. 透射电镜(TEM)、紫外-可见吸收光谱(UV-Vis)及荧光光谱(PL)研究结果表明, CdSe纳米晶呈均匀的球状颗粒, 在近红外区具有良好的吸收和荧光性能; 加入CdSe纳米晶能够有效地淬灭共轭聚合物的荧光. 在AM1.5模拟太阳光(光强为100 mW/cm2)照射下, ns-CdSe/MEH-PPV共混体系太阳电池器件性能测试结果为: 短路电流ISC为1.56 mA/cm2, 开路电压VOC为0.75 V, 填充因子FF为34.5％, 光电转换效率η为0.40%; 对于ns-CdSe/P3HT共混体系, 其ISC为1.93 mA/cm2, VOC为0.65 V, FF为38.4%, η为0.48%.     

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

Facile in situ Synthesis of Ag-Doped CdSe Supra-Quantum Dots and their Characterization

A supra-quantum dot (SQD) is a three-dimensionally assembled QD structure composed of several hundreds to thousands of QDs connected through oriented attachments. Owing to their three-dimensional interconnected structures and relatively large volumes, impurity atoms are thermodynamically more stable in SQDs than in conventional QDs. Herein, we report the facile in-situ synthesis of colloidal Ag-doped CdSe SQDs. Ag dopants were efficiently incorporated into CdSe SQDs through the three-dimensional interconnection of Ag-doped primary CdSe QDs, as confirmed by elemental analysis combined with chemical etching. Photoelectron spectroscopic studies revealed that the Ag-doped CdSe SQDs exhibit n-type doping behavior, since the valence electrons from the interstitial Ag atoms are directly donated to the lattice.     

Long-lived Single Excitons,Trions, and Biexcitons in CdSe/CdTe Type-Ⅱ Colloidal Quantum Wells

Light-harvesters with long-lived excited states are desired for efficient solar energy conversion systems. Many solar-to-fuel conversion reactions, such as H₂ evolution and CO₂ reduction, require multiple sequential electron transfer processes, which leads to a complicated situation that excited states involves not only excitons (electron-hole pairs) but also multi-excitons and charged excitons. While long-lived excitons can be obtained in various systems (e.g., semiconductor nanocrystals), multi-excitons and charged excitons are typically shorted-lived due to nonradiative Auger recombination pathways whereby the recombination energy of an exciton is quickly transferred to the third carrier on a few to hundreds of picoseconds timescale. In this work, we report a study of excitons, trions (an exciton plus an additional charge), and biexcitons in CdSe/CdTe colloidal quantum wells or nanoplatelets. The typeⅡ band alignment effectively separates electrons and holes in space, leading to a single exciton lifetime of 340 ns which is ~2 order of magnitudes longer than that in plane CdSe nanoplatelets. More importantly, the electron-hole separation also dramatically slows down Auger decay, giving rise to a trion lifetime of 70 ns and a biexciton lifetime of 11 ns, among the longest values ever reported for colloidal nanocrystals. The long-lived exciton, trion, and biexciton states, combined with the intrinsically strong light-absorption capability of two-dimensional systems, enable the CdSe/CdTe type-Ⅱ nanoplatelets as promising light harvesters for efficient solar-to-fuel conversion reactions.     

一步湿化学法合成超细金纳米线

以氯金酸(HAuCl₄)为前驱物, 油胺同时作为溶剂、表面稳定剂和还原剂, 通过简单的一步湿化学法合成超细金纳米线. 制备出的超细金纳米线不仅产量高、纯度高, 而且纵横比大, 纳米线平均直径~2 nm, 长度可达数十微米. 如果添加另一种还原剂油酸并调节油胺和油酸的体积比为1:1, 将生成直径为~9 nm的金纳米线. 通过改变反应温度和还原剂用量, 对该种超细金纳米结构的生长机制进行阐述说明: 以油胺为模板, 在油胺和一价金卤化物(AuCl)亲金键合形成的一维聚合链作用下, 被还原的金原子附着在已成核颗粒表面, 一维地生长成超细金纳米线.     

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.     

Determination of the rod-wire transition length in colloidal indium phosphide quantum rods

Colloidal InP quantum rods (QRs) having controlled diameters and lengths are grown by the solution-liquid-solid method, from Bi nanoparticles in the presence of hexadecylamine and other conventional quantum dot surfactants. These quantum rods show band-edge photoluminescence after HF photochemical etching. Photoluminescence efficiency is further enhanced after the Bi tips are selectively removed from the QRs by oleic acid etching. The QRs are anisotropically 3D confined, the nature of which is compared to the corresponding isotropic 3D confinement in quantum dots and 2D confinement in quantum wires. The 3D-2D rod-wire transition length is experimentally determined to be 25 nm, which is about 2 times the bulk InP exciton Bohr radius (of approximately 11 nm).     

Structure-property correlation of CdSe clusters using experimental results and first-principles DFT calculations

Structures and properties of CdSe quantum dots (clusters) up to a diameter of approximately 2 nm were investigated by combining experimental absorption, photoluminescence (PL), and X-ray diffraction (XRD) spectroscopies as well as ab initio DFT calculations. These CdSe clusters were nucleated and grown from solutions containing respective cadmium and selenium precursors following the hot-injection technique that allows one to obtain size-controlled CdSe clusters having PL efficiency up to 0.5. The DFT calculations were performed at the B3LYP/Lanl2dz level and followed by time-dependent TDDFT calculations to estimate n energy singlet transitions. On the basis of the results of these experimental and theoretical studies, an approach to determine whether the proposed cluster with a mean diameter of approximately 2 nm is more physically reasonable is discussed. It was shown that the minimum nucleus of a CdSe cluster consists of (CdSe)(3) with a six-membered ring and planar structure. No PL is observed for this structure. The formation of the next stable cluster depends on whether hexadecylamine (HDA) was used for the growth of the CdSe clusters. In the absence of HDA, the second cluster was found to be (CdSe)(6) characterized by a broad PL spectrum, while in the presence of HDA, it was found to be (CdSe)(n) (where n > or = 14) with a sharp PL spectrum.     

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.

Analysis of the atomic structure of colloidal quantum dots of the CdSe family: X-ray spectral diagnostics and computer modelling

Colloidal quantum dots of the CdSe family have been studied by X-ray absorption near edge structure (XANES) spectroscopy and computer modelling. CdK edge XANES spectra in colloidal quantum dots based on varisized CdSe nanoparticles have been recorded. Atomic structure of CdSe particles and also CdSe particles doped by transition metal atoms Mn and Co has been modelled based on the density functional theory. The embedding of the doping atoms is shown to result in considerable changes in the local atomic structure of CdSe particles. XANES spectra have been calculated above the CdK edge in CdSe particles, above the MnK edge in CdSe:Mn particles, above the CoK edge in CdSe:Co particles. The sensitivity of XANES spectroscopy to small changes in structural parameters of the nanoparticles of CdSe family has been demonstrated that furnishes an opportunity to apply it for the verification of atomic structure parameters around positions of cadmium and doping atoms of transition metals in quantum dots based on CdSe.     

新型磁性荧光Fe3O4-CdSe纳米复合材料的制备

以1-十八烯作为高沸点溶剂, 在磁性粒子表面沉积量子点获得新型的磁性荧光Fe₃O₄-CdSe 纳米异质结构. 首先以乙酰丙酮铁(Fe(acac)₃)为前驱体, 二苯醚为溶剂, 油酸为表面活性剂和油胺(OAm)为表面活性剂兼还原剂, 通过溶剂热法制备单分散性的Fe₃O₄ 纳米粒子. 然后以1-十八烯为高沸点溶剂, CdO 为镉源,TOP-Se为硒源, 十六胺为表面活性剂以及硬脂酸为生长促进剂和成核剂制备得到新型的Fe₃O₄-CdSe纳米异质结构. 通过透射电镜(TEM), 傅里叶变换红外(FTIR)光谱, X射线衍射(XRD)谱, X射线光电子能谱(XPS)分析仪, 振动样品磁强计(VSM), 紫外-可见(UV-Vis)光谱和光致发光(PL)等手段对Fe₃O₄-CdSe 纳米复合材料的结构和性能进行表征. 结果表明, CdSe纳米粒子成功地吸附在Fe₃O₄纳米粒子表面, 并沿着c轴生长, 形成了宽3.6 nm, 长分别为14.5 和32.5 nm的新型枣核状和钉子状的异质结构体. 这种新型的Fe₃O₄-CdSe纳米复合材料是由磁铁矿Fe₃O₄和六方形的CdSe棒状结构组成, 具有较好的荧光性能和超顺磁性. 随着CdSe棒长度的增加, 荧光吸收峰向长波方向移动. Fe₃O₄纳米粒子, 枣核状和钉子状的Fe₃O₄-CdSe纳米复合材料的饱和磁化强度分别是57.80, 40.76和31.10 emu·g^-1.     

Enhanced multiple exciton dissociation from CdSe quantum rods: the effect of nanocrystal shape

A unique ability of semiconductor nanocrystals (NCs) is the generation and accommodation of multiple excitons through either optical or electric current pumping. The development and improvement of NC-based optoelectronic devices that utilize multiple excitons requires the understanding of multiple exciton dynamics and their efficient conversion to emitted photons or external charges prior to exciton-exciton annihilation. Here, we demonstrate that significantly enhanced multiexciton dissociation efficiency can be achieved in CdSe quantum rods (QRs) compared to CdSe quantum dots (QDs). Using transient absorption spectroscopy, we reveal the formation of bound one-dimensional exciton states in CdSe QRs and that multiple exciton Auger recombination occurs primarily via exciton-exciton collision. Furthermore, quantum confinement in the QR radial direction facilitates ultrafast exciton dissociation by interfacial electron transfer to adsorbed acceptors. Under high excitation intensity, more than 21 electrons can be transferred from one CdSe QR to adsorbed methylviologen molecules, greatly exceeding the multiexciton dissociation efficiency of CdSe QDs.     

Surface-functionalization-dependent optical properties of II-VI semiconductor nanocrystals

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.