Band gap engineering of CdTe nanocrystals through chemical surface modification

We demonstrate band gap control of CdTe nanocrystals by selective surface modification using alkanethiol molecules. Both absorption and emission wavelengths can be tuned simply by mixing a dispersion of the nanocrystals with alkanethiol at room temperature, resulting in blue shifts in the optical spectra during reaction. The degree of blue shift depends on both the concentration of alkanethiols and the reaction time, thereby providing kinetic control over the emission peak wavelength of the nanocrystals in mild conditions. The observed spectral changes are suggested to be caused by a decrease in the size of the CdTe core through formation of CdTe1-x(SC10)x shells because of specific exchange of Te with alkanethiolates. The results reported herein provide a new band gap engineering scheme for semiconductor nanocrystals and offer opportunities for the design of ligand-stabilized semiconductor nanocrystals with tunable composition and optical properties.     

Size-dependent dissociation pH of thiolate ligands from cadmium chalcogenide nanocrystals

A method, pseudo steady-state titration, is introduced for determining the precipitation pH of nanocrystals coated by electron-donating ligands. CdSe nanocrystals coated with hydrophilic deprotonated thiol (thiolate) ligands were studied systematically. For comparison, CdTe and CdS nanocrystals coated with the same types of ligands were also examined. The results show that the precipitation of the nanocrystals is caused by the dissociation of the nanocrystal-ligand coordinating bonds from the nanocrystal surface. The ligands are removed from the surface due to protonation in a relatively low pH range, between 2 and 7 depending on the size, approximately within the quantum confinement size regime, and chemical composition (band gap) of the nanocrystals. In contrast, the redispersion of the nanocrystals was found to be solely determined by the deprotonation of the ligands. The size-dependent dissociation pH of the ligands was tentatively used as a means for determining the size-dependent free energy associated with the formation of a nanocrystal-ligand coordinating bond.     

水相中CdTe纳米晶的制备及其光学性质

用不同稳定剂(巯基乙酸(TGA)、巯基丙酸(MPA)、L-半胱氨酸(L-Cys)、3-巯基-1,2-丙二醇(TG))在水相中制备了CdTe纳米晶, 并用透射电子显微镜(TEM)、X射线光电子能谱(XPS)和X射线粉末衍射(XRD)等技术对其进行了表征. 研究了不同水相合成条件对CdTe纳米晶光学性质的影响, 结果表明, n(Cd):n(Te)、溶液pH值、回流时间以及稳定剂的性质, 对纳米晶的光学性质具有显著影响. 制得的CdTe纳米晶发射峰窄且对称(半高全宽达38 nm), 用不同稳定剂制备的纳米晶发光量子效率有所不同, 用不同的激发波长对纳米晶进行激发时, 发射峰并未表现出明显的移动.     

Optical absorption and valence band photoemission from uncapped CdTe nanocrystals

CdTe nanocrystals have been successfully fabricated by a mechanical alloying process. X-ray diffraction (XRD) patterns demonstrate that a single-phase CdTe compound with a zinc blende structure has been formed after ball milling elemental Cd and Te mixture powders for 27 h. The large broadening effect for the width of the {111} diffraction peak of uncapped CdTe nanocrystals on smaller size was observed in slowly scanned XRD patterns. The X-ray photoelectron spectrum was used to study the surface of the uncapped CdTe nanocrystals within both core level and valence band regions. The presence of tellurium oxide film on the surface of the uncapped CdTe nanocrystals has been detected in the X-ray photoelectron spectrum of the Te 3d core level, which was comparable to the observed amorphous oxide thin layer on the surface of uncapped CdTe nanocrystals in a high resolution transmission electron microscopy (HRTEM) image. The energy of the valence band maximum for uncapped CdTe powders blue shifts to the higher energy side with smaller particle sizes. In UV-visible optical absorption spectra of the suspension solution containing uncapped CdTe nanocrystals, the absorption peaks were locating within the ultraviolet region, which shifted toward the higher energy side with prolonged ball milling time. Both blue shifts of valence band maximum energy and absorption peaks with decreasing particle size provide a unique pathway to reveal the quantum confinement effect of uncapped CdTe nanocrystals.     

Porous CdTe nanocrystal assemblies: ligation effects on the gelation process and the properties of resultant aerogels

Highly porous CdTe nanoarchitectures (aerogels) were prepared by sol-gel assembly of discrete nanocrystals followed by supercritical CO(2) drying. CdTe nanocrystal surface functionalization (either phosphine oxide or thiolate) is found to be immaterial to oxidation induced gel formation suggesting that the standard thiolate capping procedure is not a necessary step in the gelation process. On the basis of this observation, and reduction induced dispersion of the gel network, the exposure of reactive sites and the subsequent surface oxidation reaction to form polychalcogenide linkages are key steps in the gelation mechanism. Consequently, CdTe aerogels exhibit similar physicochemical properties, regardless of original ligating functionality. The aerogels are mesoporous, with surface area >100 m(2)/g, and exhibit an optical bandgap of 1.92 eV, consistent with quantum confinement within the 3-D linked network. Photoluminescence is suppressed in the aerogels, but can be partially recovered upon heating.     

Preparation and Optical Properties of CdTe/CdO-nH2O Core/shell Nano-composites in Aqueous Solution

"The deposition of CdO?nH2O on CdTe nanoparticles was studied in an aqueous phase. The CdTe nanocrystals (NCs) were prepared in aqueous solution through the reaction between Cd2+ and NaHTe in the presence of thioglycolic acid as a stabilizer. The molar ratio of the Cd2+ to Te2- in the precursory solution played an important role in the photoluminescence of the ultimate CdTe NCs. The strongest photoluminescence was obtained under 4.0 of Cd2+/Te2- at pH?8.2. With the optimum dosage of Cd(II) hydrous oxide deposited on the CdTe NCs, the photoluminescence was enhanced greatly. The photoluminescence of these nanocomposites was kept constant in the pH range of 8.0-10.0, but dramatically decreased with an obvious blue-shifted peak while the pH was below 8.0. In addition, the photochemical oxidation of CdTe NCs with cadmium hydrous oxide deposition was markedly inhibited."     

Synthesis of Zinc Blende CdTe Nanocrystals and Facile Shape Conversion from Multipods to Dot-shaped Particles

Branched CdTe nanocrystals with zinc blende structure were directly synthesized in the early growth stage at a high initial concentration of cadmium precursor and a high molar ratio of Cd precursor to Te precuesor. Activation of the cadmium precursor by octadecylamine was found to be critical for the formation of branched CdTe nanocrystals. Furthermore, these as-prepared CdTe nanocrystals can evolve into nearly monodisperse dots through Ostwald ripening and still keep strong photoluminescence. These results manifest a new route to synthesize branch- and dot-shaped CdTe nanocrystals with zinc blende structure.     

Synthesis of CdTe/SiO_2 Composites and Their Fluorescence Properties

Thioglycolic acid(TGA)-stabilized CdTe nanocrystals(NCs) were prepared with sodium tellurite as tellurium source,which avoids the cumbersome processes associated with H2Te or NaHTe sources.Fluorescent CdTe/SiO2 composites were synthesized by a sol-gel method without the exchange of surface ligands.The phase structure of CdTe NCs was investigated by X-ray diffractometry.For comparison,some characterizations were done for both the CdTe NCs and the composites.CdTe NCs and CdTe/SiO2 composites were characterize...     

One-pot synthesis, optical property and self-assembly of monodisperse silver nanospheres

High-quality spherical silver (Ag) nanocrystals have been synthesized by using a one-pot approach, in which pre-synthesis of organometallic precursors is not required. This reaction involves the thermolysis of a mixed solution of silver acetate and n-dodecanethiol in a non-coordinating organic solvent. The size of the as-obtained Ag nanospheres can be controlled by adjusting the reaction time, reaction temperature and the amount of silver acetate added. The growth and nucleation process of the resultant Ag nanospheres have been studied by employing UV-vis absorption spectra and transmission electron microscopy (TEM) images. Furthermore, these Ag nanospheres have good self-assembly behaviors, and they are easily self-assembled into two- or three-dimensional superlattice structures due to the bundling and interdigitation of thiolate molecules adsorbed on Ag nanospheres. This one-pot synthetic procedure is simple and highly reproducible, which may be extended to prepare other noble-metal nanocrystals.     

Stable Water‐dispersed CdTe Nanocrystals Dependent on Stoichiometric Ratio of Cd to Te Precursor

The improved properties of CdTe nanocrystals (NCs) synthesized by hydrothermal method were introduced. The experimental results indicated that the NCs properties could be dramatically influenced by means of changing Cd‐to‐Te molar ratio (the molar ratio of CdCl₂ and NaHTe in the precursor) of the MPA‐capped CdTe NCs. With the increase of the ratio from 2:1 to 10:1, the formation time of near‐infrared‐emitting CdTe NCs was shortened. In particular, high Cd‐to‐Te molar ratio brought about MPA‐capped CdTe NCs of superior radical oxidation‐resistance and photostability. As a result, the optimum ratio was found to be 8:1 or 10:1 in the study in order to efficiently attain stable, water‐dispersed CdTe NCs.     

电化学法合成强荧光CdTe量子点

采用电化学方法产生的H2Te为碲源(Te2-),快速合成了水溶性强荧光的CdTe量子点.该方法具有操作简单、安全、快速廉价和可大量制备等优点.合成过程中考察了合成温度,pH值和配体比例对制备CdTe量子点的影响.在最优化的实验条件下,电化学方法合成的巯基丙酸配位的CdTe荧光量子产率可达到55％;通过紫外可见光谱(UV...     

Synthesis of CdTe nanocrystals using Te nanorods as the Te source and the formation of microtubes with red fluorescence

Here, we report an alternative route to the preparation of highly luminescent CdTe nanocrystals (NCs) using Te nanorods instead of freshly prepared NaHTe as the Te source via a one-pot route under hydrothermal conditions. Furthermore, microtubes with red fluorescence were generated via simply aging the above CdTe NC suspension under ambient conditions.     

Synthesis of CdTe nanocrystals through program process of microwave irradiation

A novel method, the program process of microwave irradiation (PPMI), which commendably integrates good qualities of microwave irradiation and a programmed heating process, is presented to synthesize high-quality CdTe nanocrystals in aqueous solution. Microwave irradiation, which acts as the heating mode of PPMI, is highly favorable for a narrow size distribution and low concentration of surface defects of nanocrystals. On the other hand, two correlative processes (the first process and the second process) are utilized in PPMI to actualize the programmed heating process, which is an effective strategy to improve the quality of nanocrystals. Thus, a series (diameters approximately 2-4 nm) of highly luminescent (PLQY approximately 30-68%) CdTe nanocrystals were rapidly prepared (reaction time approximately 1-30 min) in aqueous phase through PPMI.     

通过静电相互作用制备CdTe/SiO2纳米复合物

以四乙氧基硅烷(TEOS)为原料,在乙醇相中制备了尺寸为80nmSiO₂纳米粒子,用3-氨基丙基三乙氧基硅烷对SiO₂纳米粒子进行了氨基化.以巯基乙酸为配体,在水相中合成了CdTe纳米晶.通过静电相互作用,CdTe纳米晶被吸附到氨基化的二氧化硅球的表面上.研究了两种粒子复合后引起的CdTe纳米晶发光光谱的变化.     

Synthesis of Aqueous CdTe Nanocrystals with High Efficient Blue‐Green Emission of Exciton

As one of the most popular nanocrystals (NCs), aqueous CdTe NCs have very weak green emission under conventional synthesis conditions. In this work, we report the first example of blue‐emitting CdTe NCs directly synthesized in aqueous solution by slowing down the growth rate after nucleation. The key for the synthesis is the optimization of NC growth conditions, namely pH range of 7.5 to 8.5, TGA/Cd ratio of 3.6, Cd/Te ratio of 10, and Te concentration of 2×10^?5 mol/L, to get a slow growth rate after nucleation. The as‐prepared blue‐emitting CdTe NCs have small size (as small as 1.9 nm) and bright emission [with 4% photoluminescence quantum yield (PL QY) at 486 nm and 17% PLQY at 500 nm]. Transmission electron microscopy (TEM) images of the as‐prepared CdTe show monodispersed NCs which exhibit cubic zinc blend structure. Moreover, time‐resolved PL decay and X‐ray photoelectron spectroscopy (XPS) results show the as‐prepared NCs have better surface modification by ligand, which makes these luminescent small CdTe NCs have higher photoluminescence quantum yield, compared with NCs synthesized under conventional conditions.     

Solid state NMR studies of photoluminescent cadmium chalcogenide nanoparticles

Solid state (113)Cd, (77)Se, (13)C and (31)P NMR have been used to study a number of Cd chalcogenide nanoparticles synthesized in tri-n-octyl-phosphine (TOP) with different compositions and architectures. The pure CdSe and CdTe nanoparticles show a dramatic, size-sensitive broadening of the (113)Cd NMR line, which can be explained in terms of a chemical shift distribution arising from multiple Cd environments. From (13)C NMR, it has been discovered that TOP, or its derivatives such as TOPO (trioctylphosphine oxide), is rapidly moving about the surface of the nanoparticles, indicating that it is relatively weakly bound as compared to other materials used as surface ligands, such as hexadecylamine. (31)P NMR of the nanoparticles shows at least five species arising from coordination of the ligands to different surface sites. (113)Cd NMR of CdSeTe alloy and layered nanoparticles has provided crucial information which, in conjunction with results from other techniques (especially optical characterization), has made it possible to develop a detailed picture of the composition and structure of these materials: (i) a true CdSeTe homogeneous alloy nanoparticle, (ii) a nanoparticle segregated into an alloy core region rich in Te, with a CdSeTe (close to 1 : 1 Se : Te) alloy shell and (iii) a CdSe/CdTe/CdSe layered nanoparticle in which the CdTe layer contains a small amount of Se and which forms a Quantum Dot Quantum Well (QDQW) system. The results demonstrate that solid state NMR is a vital tool in the arsenal of characterisation techniques available for nanomaterials.     

Interaction of CdTe nanocrystals with thiol-containing amino acids at different pH: a fluorimetric study

3-Mercaptopropionic acid (MPA)-capped CdTe nanocrystals (NCs) were synthesized in aqueous medium, and their interaction with cysteine (Cys) and homocysteine (Hcy) was studied by steady-state and time-resolved fluorescence spectra at different pH. At 6.4?<?pH?<?8.0, the fluorescence of CdTe NCs can be effectively enhanced by Cys and Hcy. While pH?>?9.6, only Cys quenches the fluorescence of the CdTe NCs, no fluorescence changes are observed for Hcy. Mechanism study shows that these pH manipulating fluorescence responses can be attributed to the following two reasons: first, both the thiol–thiolate equilibrium of Cys (Hcy) and the number of undercoordinated NCs surface sites capped with dual coordinated ligands are strong pH-dependent; second, different thiol-containing amino acids, with different redox energy level, can lead to distinguishable fluorescence responses of NCs. Based on these unique fluorescence responses, the possibilities of developing a sensitive detecting technique for Cys/Hcy and Cys through pH modulation can be explored.     

聚合物与纳米晶的杂化及性能优化

聚合物-纳米晶杂化材料因结合了有机和无机材料的优点而逐渐地受到了人们普遍的关注,聚合物为纳米晶的形成与生长提供了优良的环境,纳米晶的引入同样也增加和强化了聚合物的功能特性.如聚硫代氨基甲酸酯与TiO2杂化的高折射率薄膜,该薄膜不仅保持了原有的性能,而且有较高的折射率.此外,还有许多不同纳米粒子与不同聚合物的杂化体系.如...     

Oligomerization of cadmium chalcogenide nanocrystals into CdTe-containing superlattice chains

Hierarchically-assembled binary cadmium chalcogenide (CdX, X = S, Se, Te) oligomers were engineered. CdTe precursor injections into a solution containing seeded nanocrystals provided longer oligomers possessing alternating and phase-separated CdX and CdTe constituents in the same wire, like an alternating copolymer.     

Preparation of monodisperse CdTe nanocrystals-SiO2 microspheres without ligands exchange

Fluorescent CdTe-SiO₂ composite microspheres were prepared by a sol–gel method without the exchange of surface ligands for the first time. We loaded CdTe nanocrystals (NCs) into the matrix of silica microspheres during the formation of composite spheres. In contrast to CdTe NCs in aqueous solutions, CdTe NCs in the composite microspheres revealed better stability while their fluorescence properties were retained due to the confined effects of silica matrix. In addition, we also investigated the dependency of properties of these composite spheres on such important synthesis factors as pH value, concentration and stabilizers during experiment procedure in details.