Water-soluble CdTe Nanocrystals Research Articles

The Ratio of NaBH$_{4}$ in the Synthesis of Water-soluble CdTe Nanocrystals

The Ratio of NaBH$_{4}$ in the Synthesis of Water-soluble CdTe Nanocrystals

Flexible and fragmentable tandem photosensitive nanocrystal skins.

We proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 μW cm(-2) at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.

Controlled optical properties of water-soluble CdTe nanocrystals via anion exchange

We report a study on anion exchange reaction of CdTe nanocrystals with S2− in aqueous solution under ambient condition. We found that the optical properties of CdTe nanocrystals can be well tuned by controlling the reaction conditions, in which the reaction temperature is crucially important. At low reaction temperature, the product nanocrystals showed blue-shifts in both absorption and PL spectra, while the photoluminescence quantum yield (PLQY) was significantly enhanced. When anion exchanges were carried out at higher reaction temperature, on the other hand, obvious red shifts in absorption and PL spectra accompanied by a fast increase followed by gradual decrease in PLQY were observed. On variation of S2− concentration, it was found that the overall kinetics of Te2− for S2− exchanges depends also on [S2−] when anion exchanges were performed at higher temperature. A possible mechanism for anion exchanges in CdTe NCs was proposed.

Enhanced photoluminescence of water-soluble CdTe nanocrystals induced by light and pyridine

Abstract We report an efficient way to improve the photoluminescence (PL) of water-soluble CdTe nanocrystals (NCs) by irradiating the NCs in the presence of pyridine. Under illumination, continuous increases in PL of CdTe NCs were observed. The maximum quantum yield reached up to 41% was obtained, showing five times higher than that of the as prepared parent CdTe NCs. While keeping the illuminated CdTe NCs in dark, further increase in PL quantum yield to over 60% was observed. PL decay measurements revealed that the PL enhancement is associated with the increase of the average lifetime of CdTe NCs. The effects of light wavelength and intensity, pyridine derivatives as well as oxygen on the PL of CdTe NCs were investigated, and a competition mechanism between photodegradation and surface passivation of CdTe NCs induced by illumination and pyridine was suggested.

Versatile Self-Assembly of Water-Soluble Thiol-Capped CdTe Quantum Dots: External Destabilization and Internal Stability of Colloidal QDs

In this paper, we report on the versatile self-assembly of water-soluble thiol-capped CdTe quantum dots (QDs), nanoparticles (NPs), or nanocrystals induced by L-cysteine (L-Cys). Major efforts are focused on the control of the self-organization of QDs into nanosheets (NSs), for example, by altering the solution pH and the QD size. The as-prepared nanosheets exhibit bright photoluminescence (PL) and retain the size-quantized properties of initial CdTe QDs, since they are actually formed by a 2D network of assembled QDs. By optical techniques, TEM, EDX, powder XRD, etc., it is found that the unique L-Cys-induced external destabilization is responsible for the template-free self-organization process, with the further assistance of the specific NP-NP interactions. And the internal chemical stability of initial CdTe QDs also is proven for the first time to play an important role. These results help to enhance the current understanding about the mechanism for the destabilization of colloidal NPs and their self-assembly behavior.

Experimental and theoretical investigations of the ligand structure of water-soluble CdTe nanocrystals

The unique optical properties, such as size-tunable absorption and emission, caused semiconductor nanocrystals to attract a great deal of interest for recent technological developments. For the evaluation of semiconductor nanocrystals as new materials for various applications like optoelectronic devices, knowledge of the structure-property relationships is indispensable, but still presents a challenge. Here, we address these challenges for thioglycolic acid-capped CdTe nanocrystals with a focus on the quantification of thiol ligands, identification of the ligand shell structure and their influence on the optical properties of these nanocrystals. We present the use of a simple analytical technique, the Ellman's test, and ICP-OES analysis for the study of the surface chemistry of these nanomaterials. Together with theoretical calculations, the results of these studies show the strong influence of the amount of Cd-thiolates present in the ligand shell on the concentration-dependent emission properties, thereby providing the basis for a better understanding of the chemical nature of the NC-ligand interface. In this context, the present work contributes to the establishment of a clearer picture and better control of the surface chemistry, which will provide the basis for the design of highly emitting nanocrystals and the prediction of their applicability.

An effective and simple oxygen nanosensor made from MPA-capped water soluble CdTe nanocrystals

CdTe semiconductor nanocrystals (NCs) with 3-mercaptopropionic acid as the ligand exhibit a reversible response towards inter-switching oxygen and argon environments. The photoluminescence response is investigated at multiple oxygen concentrations, NC coverage and excitation intensities, in which all conditions exhibit full recovery upon exposure to flowing argon. The CdTe NC’s large surface-to-volume ratio results in high sensitivity towards oxygen molecules with significant photoluminescence quenching at a concentration of 40 ppm. This suggests a novel approach to the creation of simple, inexpensive and ultrasensitive oxygen nanosensors.

Facile one-step photochemical synthesis of water soluble CdTe(S) nanocrystals with high quantum yields

This paper describes a successful synthesis of water soluble CdTe(S) nanocrystals (NCs) with high quantum yields (QYs) via a mild photochemical route in one step with thioglycolic acid as the stabilizer and sulfur source. The precursor solution of mixed cadmium chloride, freshly prepared sodium hydrogen telluride and thioglycolic acid was subjected directly to UV irradiation using a 500 W high-pressure mercury lamp under the protection of N2. NCs with a small particle size (2.2 nm) and a high photoluminescence quantum yield (up to 80% at room temperature) were formed within 25 min. The NCs were characterized by UV-vis absorption and fluorescence spectroscopy, transmission electron microscopy, powder X-ray diffraction and energy-dispersive X-ray spectroscopy to evaluate their structure, size and composition. The photochemically prepared NCs were compared with CdTe NCs prepared by conventional hydrothermal synthesis and CdTe/CdS core/shell NCs obtained by post-preparative photochemical passivation. The effects of UV irradiation intensity, illumination time, temperature and concentration of Te2− on the optical properties of NCs were studied in detail.

Aqueous-Solution-Processed Hybrid Solar Cells from Poly(1,4-naphthalenevinylene) and CdTe Nanocrystals

Poly(1,4-naphthalenevinylene), prepared from a water-soluble precursor, was used to fabricate hybrid solar cells by blending with water-soluble CdTe nanocrystals (NCs) to act as the photoactive layer. In composites with CdTe NCs as the electron acceptors in a bulk heterojunction configuration, the devices exhibited a short-circuit current density of -6.14 mA/cm(2), an open-circuit voltage of 0.44 V, a fill factor of 0.32, and a power conversion efficiency of 0.86% under AM1.5G conditions. Because the devices were fabricated from water-soluble materials, the procedure was generally simple and environmentally friendly in comparison to the conventional devices fabricated from oil-soluble materials.

CdTe Nanocrystals Capped with a Tetrazolyl Analogue of Thioglycolic Acid: Aqueous Synthesis, Characterization, and Metal-Assisted Assembly

A novel ligand, 5-mercaptomethyltetrazole, is applied to the direct colloidal synthesis of highly luminescent (quantum yield of up to 60%) water-soluble CdTe nanocrystals. In the synthesis the 5-mercaptomethyltetrazole behaves analogously to the widely used thioglycolic acid, providing the additional advantage of solubility of the resulting nanocrystals in methanol. Moreover, the 5-mercaptomethyltetrazole-capped nanocrystals possess the unique ability to reversibly form fine 3D networks (hydrogels) upon the addition of metal salts.

Efficient light harvesting in hybrid CdTe nanocrystal/bulk GaAs p-i-n photovoltaic devices

A hybrid colloidal CdTe nanocrystal/bulk GaAs p-i-n heterostructure is demonstrated to have potential for highly efficient light harvesting photovoltaic devices. An array of rectangular channels is fabricated on the surface of the GaAs heterostructure penetrating through its active layer and subsequently filled with water soluble CdTe nanocrystals emitting in the near infrared. Photogenerated carriers in the highly absorbing colloidal nanocrystals are efficiently transferred by means of nonradiative energy transfer to the patterned heterostructure possessing high carrier mobility and converted to electrical current. A threefold enhancement of both photocurrent and monochromatic power conversion efficiency has been achieved.

Ultrafast synthesis of water-soluble nanocrystals by the chemical aerosol flow method

Water-soluble CdTe nanocrystals are ultrafast synthesized in less than 10 s via the chemical aerosol flow method.

Fabrication of fluorescent microparticles by doping water-soluble CdTe nanocrystals into calcium carbonate for monitoring intracellular uptake

3-Mercaptopropyl acid (MPA) capped CdTe quantum dots (QDs) were embedded into CaCO 3 microparticles with a size of 1.4–4.4 μm by addition of the QDs into Ca(NO 3) 2 solution during a mineralization process. Compared to the parent QDs, about 1/7–1/3 photoluminescence efficiency of the embedded QDs was preserved, enabling the CaCO 3(CdTe) particles visible under UV irradiation. The structure and morphology of the CaCO 3(CdTe) particles were characterized by X-ray diffraction (XRD), UV–vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM). Protected by the CaCO 3 particles, the QDs in the composites were more stable against long term storage, UV irradiation and cell culture medium containing serum. The CaCO 3(CdTe) particles could be internalized into live cells, human liver cancer cells (HepG2), for example, and most of which distributed in the lysosomes as revealed by confocal microscopy. Also the CaCO 3(CdTe) particles had low cytotoxicity in comparison with the parent CdTe QDs.

Energy Transfer in Solution-Based Clusters of CdTe Nanocrystals Electrostatically Bound by Calcium Ions

Divalent positively charged Ca2+ ions acting as electrostatic chelating linkers were used to induce clustering of water-soluble CdTe nanocrystals through coordination to the negatively charged carboxylic acid groups of their ligands. An efficient Förster resonant energy transfer (FRET) was responsible for changes in the emission spectra and the luminescence decay times of CdTe nanocrystal clusters composed of particles of two and three different sizes. FRET was also observable in nominally single-sized samples with intrinsically broadened size distributions. Excitation energy transfer toward acceptor nanocrystals in binary clusters resulted in a 77% increase of the acceptor emission intensity at room temperature. Time-resolved fluorescence measurements revealed that at least part of the energy transfer involves long-lived intermediate trap states and the recycling of this trapped excitation energy by transfer to red-emitting nanocrystals.

Synthesis of highly stable dihydrolipoic acid capped water-soluble CdTe nanocrystals

To prevent the dissociation of organic ligands from the inorganic core and accordinglyenhance colloidal stability, is the prerequisite for the further application of nanocrystals indifferent areas. Dihydrolipoic acid (DHLA) appears very promising for the stabilization andthe further functionalization of CdTe nanocrystals because DHLA is characterized by acarboxylic acid group and two thiol functions, through which simultaneous anchorage ontoa nanoparticle’s surface is possible. For the first time, we synthesized highly luminescentCdTe nanocrystals (maximum quantum yield of 42%) in aqueous solution by the reaction ofCd2+ and NaHTe using DHLA as capping reagent. The influence of various experimentalvariables, including reaction temperature, ligand-to-Cd ratio and pH value as well asTe-to-Cd ratio, on the growth rate and optical properties of the obtained CdTenanocrystals have been systematically investigated. Furthermore, the stability of thedithiol DHLA capped CdTe nanocrystals was compared with the monothiol ligandsthioglycolicacid (TGA) and mercaptopropionic acid (MPA) capped nanocrystals.Experimental results indicate that the DHLA capped nanoparticles show better chemicaland thermal stability than those capped by TGA or MPA. The mechanism for thehigh-quality optical properties and the good stability of the resulting CdTe nanocrystals isalso primarily elucidated.

Synthesis and Optical Properties of Water-Soluble CdTe Nanocrystals

CdTe nanocrystals were synthesized in aqueous solution with different stabilizers (L-cysteine hydrochloride (L-Cys), 3-mercaptopropionic acid (MPA), 1-thioglycerol (TG), and thioglycolic acid (TGA)) by a simple route, and were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and X-ray diffraction pattern (XRD). The effect of aqueous synthesis conditions on the optical properties of CdTe nanocrystals was investigated and it was found that the molar ratio of Cd/Te, pH value, refluxing time, and different stabilizers significantly affected the optical properties of CdTe nanocrystals. The obtained CdTe nanocrystals exhibited a favorable narrow emission band under different conditions. The photoluminescence (PL) peaks and absorption peaks could be tuned by changing the synthesis conditions of CdTe nanocrystals. Different PL quantum yields (QY) were obtained using different stabilizers. The PL peak position of CdTe nanocrystals did not show significant shift under different excitation wavelengths.

Study of influence of metal ions on CdTe/H 2O 2 chemiluminescence

Water-soluble CdTe nanocrystals (NCs) were synthesized and applied to study the effect of metal ions on chemiluminescence (CL) of the CdTe/H 2O 2 system. The effects of experimental factors, such as pH, particle sizes of CdTe and reagent concentration, on kinetics curve and CL intensity of the CdTe/H 2O 2 system were examined. The results showed that under the optimal conditions Ba 2+, Ca 2+, Fe 2+, Pb 2+ and Cu 2+ enhanced the CL intensity, and Cr 3+, Ni 2+, Zn 2+ and Ag + inhibited the CL intensity.

Preliminary studies of application of CdTe nanocrystals and dextran–Fe 3O 4 magnetic nanoparticles in sandwich immunoassay

Background The favorable properties of water-soluble CdTe nanocrystals as novel biological luminescent label over conventional fluorescent probes have attracted considerable interest. The magnetic separation technique has widely been applied to various aspects in biotechnology in recent years. In this paper, we made use of CdTe nanocrystals and dextran–Fe 3O 4 magnetic nanoparticles for fluorescence immunoassay. Methods The CdTe nanocrystals and dextran–Fe 3O 4 magnetic nanoparticles were applied to immunoassay for the determination of human immunoglobulin G (HIgG). A rabbit anti-HIgG antibody (primary antibody) was immobilized on magnetic nanoparticles, which was used as a solid support. A sheep anti-HIgG antibody (secondary antibody) was attached to the surface of the CdTe nanocrystals via electrostatic interaction. The immunoassay was based on a sandwich immunoreaction of primary antibody on the magnetic nanoparticles, HIgG (or serum sample), and the secondary antibody labeled with CdTe nanocrystals. Results The CdTe label was compared with a fluorescein isothiocyanate (FITC) label. The CdTe is an order of magnitude more sensitive than the FITC. The immunoassay method was applied to determining the HIgG in practical samples and the results obtained are in good agreement with those obtained by nephelometry. Conclusion This technique may be applied in many types of antibody–antigen system.

The selective synthesis of water-soluble highly luminescent CdTe nanoparticles and nanorods: The influence of the precursor Cd/Te molar ratio

In this report, we initially systematically investigated the influence of the precursor Cd/Te molar ratio on the morphology of water-soluble thiol-stabilized CdTe nanocrystals. By only changing the precursor Cd/Te molar ratio, highly luminescent CdTe nanoparticles (NPs) and nanorods (NRs) can be prepared selectively in the presence of the same concentration single ligand ( l-cysteine or thioglycolic acid) system. A high precursor Cd/Te molar ratio leads to isotropic spherical growth, whereas a low Cd/Te molar ratio promotes the linear self-assembly of NPs into NRs. Thus, a new efficient strategy has been developed in aqueous phase to prepare selectively highly luminescent dot- or rod-shaped CdTe nanocrystals in single ligand system. Then, we further explored the influence of the properties of initial CdTe dispersions with different luminescence maxima on the formation of NRs. The experiment results revealed that the formation of CdTe NRs is also dependent on the properties of initial CdTe dispersions. CdTe NPs with short wavelength emission (∼520–550 nm) can self-assemble directly into high quality CdTe NRs after storage at room temperature.

Direct observation of resonant energy transfer between quantum dots of two different sizes in a single water droplet

Water soluble CdTe nanocrystals (NCs) of two different sizes capped by thioglycolic acid were synthesized and were dispersed in pure water. We have observed that the emission color of CdTe NCs has been changed in a single droplet of water during the evaporation of the solvent. This is attributed to Förster resonance energy transfer between CdTe NCs, when their concentration becomes high enough to ensure close proximity. By combining our results of photoluminescence with quantitative analysis we conclude that the coupling between different sized CdTe NCs via long-range dipole-dipole interactions was enhanced in aqueous solution due to Brownian motion.