Orbital susceptibilities of PbSe quantum dots

Different sizes of three-dimensional PbSe quantum dots have been synthesized for the study of orbital magnetic susceptibilities. Two types of orbital susceptibilities have been found, including the Curie susceptibility and finite-size corrections to the Landau susceptibility. The Curie term of a quantum dot manifests itself in the temperature dependence of magnetic susceptibility at low temperatures, while the field dependence of differential susceptibility at high temperatures shows finite-size corrections to the Landau susceptibility. Both of the two kinds of orbital susceptibility, estimated per quantum dot, show linear dependence on the size.     

Excited-state relaxation in PbSe quantum dots

In solids the phonon-assisted, nonradiative decay from high-energy electronic excited states to low-energy electronic excited states is picosecond fast. It was hoped that electron and hole relaxation could be slowed down in quantum dots, due to the unavailability of phonons energy matched to the large energy-level spacings ("phonon-bottleneck"). However, excited-state relaxation was observed to be rather fast (< or =1 ps) in InP, CdSe, and ZnO dots, and explained by an efficient Auger mechanism, whereby the excess energy of electrons is nonradiatively transferred to holes, which can then rapidly decay by phonon emission, by virtue of the densely spaced valence-band levels. The recent emergence of PbSe as a novel quantum-dot material has rekindled the hope for a slow down of excited-state relaxation because hole relaxation was deemed to be ineffective on account of the widely spaced hole levels. The assumption of sparse hole energy levels in PbSe was based on an effective-mass argument based on the light effective mass of the hole. Surprisingly, fast intraband relaxation times of 1-7 ps were observed in PbSe quantum dots and have been considered contradictory with the Auger cooling mechanism because of the assumed sparsity of the hole energy levels. Our pseudopotential calculations, however, do not support the scenario of sparse hole levels in PbSe: Because of the existence of three valence-band maxima in the bulk PbSe band structure, hole energy levels are densely spaced, in contradiction with simple effective-mass models. The remaining question is whether the Auger decay channel is sufficiently fast to account for the fast intraband relaxation. Using the atomistic pseudopotential wave functions of Pb(2046)Se(2117) and Pb(260)Se(249) quantum dots, we explicitly calculated the electron-hole Coulomb integrals and the P-->S electron Auger relaxation rate. We find that the Auger mechanism can explain the experimentally observed P-->S intraband decay time scale without the need to invoke any exotic relaxation mechanisms.     

Control of particle size distribution of CdS quantum dots in gel matrix

A novel sol-gel process has been developed to prepare nano-sized CdS quantum dots to improve the nonlinear optical properties. A bifunctional ligand, 3-aminopropyl triethoxysilane H₂N(CH₂)₃Si(OC₂H₅)₃, was used to disperse the Cd²⁺ ions in the gel solution. The CdO and CdS particles were observed by transmission electron microscope (TEM). The size of CdS microcrystallites with concentrations up to 13 wt.% in SiO₂ gel matrix was found to be in the range of 2–4 nm with a very sharp size distribution. A well-defined absorption edge was observed in the absorption spectrum.     

Air-stable PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystal quantum dots and their applications

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.     

Alloyed semiconductor quantum dots: tuning the optical properties without changing the particle size

Alloyed semiconductor quantum dots (cadmium selenium telluride) with both homogeneous and gradient internal structures have been prepared to achieve continuous tuning of the optical properties without changing the particle size. Our results demonstrate that composition and internal structure are two important parameters that can be used to tune the optical and electronic properties of multicomponent, alloyed quantum dots. A surprising finding is a nonlinear relationship between the composition and the absorption/emission energies, leading to new properties not obtainable from the parent binary systems. With red-shifted light emission up to 850 nm and quantum yields up to 60%, this new class of alloyed quantum dots opens new possibilities in band gap engineering and in developing near-infrared fluorescent probes for in vivo molecular imaging and biomarker detection.     

Pushing the band gap envelope: mid-infrared emitting colloidal PbSe quantum dots

Efficient mid-infrared sources are of considerable general interest for gas analysis, remote sensing, and atmospheric monitoring, but existing technologies are limited. Here, we report the synthesis of the first colloidal QDs having photoluminescence (PL) in the mid-infrared. We show particle-size-tunable mid-infrared emission for large (10-17 nm), but quantum-confined, colloidal PbSe QDs, with efficient, narrow-bandwidth PL at energies as low as 0.30 eV (4.1 mum). Applying two new synthetic routes, we have achieved fine control of QD size and size distribution, allowing us to provide the first systematic correlation of QD size with PL energy for PbSe QDs emitting at wavelengths longer than 2 mum, results which are compared with a literature model. For the entire spectral range reported, we provide measured quantum yields in emission, showing a marked decrease with increasing QD size, for which we include a possible explanation. Finally, we present very promising preliminary results for overcoating PbSe with CdSe, a wider-gap semiconductor. We show PL enhanced by approximately 6-fold for such core/shell samples.     

Electrochemical biosensing for dsDNA damage induced by PbSe quantum dots under UV irradiation

<正>An electrochemical sensor for the detection of the natural double-stranded DNA(dsDNA) damage induced by PbSe quantum dots(QDs) under UV irradiation was developed.The biosensing membranes were prepared by successively assembling 3- mercaptopropionic acid,polycationic poly(diallyldimethyl ammonium) and dsDNA on the surface of the gold electrode.Damage of dsDNA was fulfilled by immersing the sensing membrane electrode in PbSe QDs suspension and illuminating it with an UV lamp. Cyclic voltammetry was utilized to detect dsDNA damage with Co(phen)_3~(3+) as the electroactive probe.The UV irradiation,Pb~(2+) ions liberated from the PbSe QDs under the UV irradiation and the reactive oxygen species(ROS) generated in the presence of the PbSe QDs also under the UV irradiation were the three factors of inducing the dsDNA damage.The synergistic effect of the three factors might dramatically enhance the damage of dsDNA.This electrochemical sensor provided a simple method for detecting DNA damage,and may be used for investigating the DNA damage induced by other QDs.     

PbSe quantum dots: Preparation in a high boiling point solvent and characterization

A novel approach to preparing PbSe quantum dots in a high-boiling-point solvent (paraffin liquid) was studied. PbSe quantum dots obtained were transferred from the organic phase to aqueous phase. The PbSe samples were characterized by transmission electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis, which demonstrated that high-quality PbSe quantum dots with regular shape and uniform size were prepared. The mechanism of PbSe quantum dot formation was briefly discussed. The text was submitted by the authors in English.     

An insight into the surface engineering of colloidal PbSe quantum dots for polymer hybrid photovoltaic applications

Journal of Sol-Gel Science and Technology - In this article, the beneficial effect of surface passivation rendered by the amalgamation of co-ordinating tri-n-octylphophine (TOP), oleylamine (OLA),...     

Determination of the size of quantum dots by fluorescence spectroscopy

There has been a lack of quick, simple and reliable methods for determination of nanoparticle size. An investigation of the size of hydrophobic (CdSe) and hydrophilic (CdSe/ZnS) quantum dots was performed by using the maximum position of the corresponding fluorescence spectrum. It has been found that fluorescence spectroscopy is a simple and reliable methodology to estimate the size of both quantum dot types. For a given solution, the homogeneity of the size of quantum dots is correlated to the relationship between the fluorescence maximum position (FMP) and the quantum dot size. This methodology can be extended to the other fluorescent nanoparticles. The employment of evolving factor analysis and multivariate curve resolution-alternating least squares for decomposition of the series of quantum dots fluorescence spectra recorded by a specific measuring procedure reveals the number of quantum dot fractions having different diameters. The size of the quantum dots in a particular group is defined by the FMP of the corresponding component in the decomposed spectrum. These results show that a combination of the fluorescence and appropriate statistical method for decomposition of the emission spectra of nanoparticles may be a quick and trusted method for the screening of the inhomogeneity of their solution.     

Subnanometer size uncapped quantum dots via electroporation of synthetic vesicles

The very rapid, usually diffusion-controlled, self-aggregation of nascent molecules of semiconductors (MX) or metals (M) in solution represents an experimental challenge for arresting the growth of the particles at a desired size. Unfortunately, the typical remedy used, namely capping of the clusters with a protective coating, alters their intrinsic electronic and optical properties. An additional defect of capping's virtue is that it prevents the observation of further cluster growth—which is especially important in the subnanometer (molecular) size regime, where particle growth is associated with dramatic changes in structure, surface states, and transition energy.

We have developed a novel method for the preparation of subnanometer size uncapped quantum dots, which also allows the monitoring of their growth up to several hundreds of nanometer in diameter. The essence of the method is the initial encapsulation of the metal ion (M⁺) in synthetic vesicles (liposomes) and the placement of the anion (X⁻) in the bulk solution. Exposure of the suspension to a rectangular pulse of a high-voltage homogenous electric field E of suitable intensity and duration causes the formation of transient pores in the vesicle's bilayer (electroporation). A fraction of the metal ions that are ejected through the pores react with the anions in the bulk, and the freshly created monomers (MX) adsorb on the exterior surface of the vesicle. On the vesicle surface, the self-aggregation is slowed down to the hour and day timescales which allows for convenient spectral monitoring of the growth of the clusters.

The discussion will focus on the behavior of vesicles in an electric field, the mechanism of electroporation, and our experimental and density functional theoretical findings of previously unobserved, unusual spectroscopic properties of subnanometer size AgBr, CdS, PbS, ZnS and gold quantum dots.     

Precise size control and synchronized synthesis of six colors of CdSe quantum dots in a slow-increasing temperature gradient

The present study describes a simultaneous and highly reproducible large-scale synthesis of six (and more) colors of size-homogeneous and highly luminescent CdSe quantum dots in a single reaction, controlled by a slow-increasing temperature gradient. The described protocol allows a precise control and a synchronized isolation of aliquots of CdSe nanocrystals with defined sizes, avoiding disturbance of the growth of nanocrystals (existing in the reaction mixture) to the isolation of the next aliquot. The obtained quantum dot fractions are of high quality (in 95% size-homogeneous) and have sharp photoluminescence spectra (fwhm approximately 30 nm), quantum yields of 45-70% (in organic solvent), and a lack of aggregation in organic solvents. The method is environmentally friendly as it ensures almost complete utilization of the precursors and productive yield approximately 95%.     

Minimizing the hydrodynamic size of quantum dots with multifunctional multidentate polymer ligands

We report a new strategy to minimize the hydrodynamic size of quantum dots (QDs) and to overcome their colloidal stability and photobleaching problems based on the use of multifunctional and multidentate polymer ligands. A novel finding is that a balanced composition of thiol (-SH) and amine (-NH 2) coordinating groups grafted to a linear polymer chain leads to highly compact nanocrystals with exceptional colloidal stability, a strong resistance to photobleaching, and high fluorescence quantum yields. In contrast to the standing brushlike conformation of PEGylated dihydrolipoic acid molecules, mutlidentate polymer ligands can wrap around the QDs in a closed "loops-and-trains" conformation. This structure is highly stable thermodynamically and is responsible for the excellent colloidal and optical properties. We have optimized this process for the preparation of ultrastable CdTe nanocrystals and have found the strategy to be broadly applicable to a wide range of nanocrystalline materials and heterostructures. This work has led to a new generation of bright and stable QDs with small hydrodynamic diameters between 5.6 and 9.7 nm with tunable fluorescence emission from the visible (515 nm) to the near-infrared (720 nm). These QDs are well suited for molecular and cellular imaging applications in which the nanoparticle hydrodynamic size must be minimized.     

Multicolor electrochemiluminescence of core-shell CdSe@ZnS quantum dots based on the size effect

Multicolor electrochemiluminescence (ECL) of semiconductor nanocrystals tuned by size effect has been successfully achieved using quantum dots (QDs) with core-shell structure for the first time. It would open a new way and provide a guidance for design and preparation of stable and strong multicolor ECL emitters for simultaneous multicomponent analysis application.     

pH-sensitive quantum dots

We have designed organic ligands able to adsorb on the surface of CdSe-ZnS core-shell quantum dots and switch the luminescence of the inorganic nanoparticles in response to hydroxide anions. These compounds incorporate a [1,3]oxazine ring within their molecular skeleton, which reacts with the nucleophilic hydroxide anion to generate a 4-nitrophenylazophenolate chromophore. The chromogenic transformation activates an energy transfer pathway from the quantum dot to the adsorbed chromophores. As a result, the luminescence intensity of the coated nanoparticles decreases significantly in the presence of hydroxide anions. In fact, this mechanism can be exploited to probe the pH of aqueous solutions. Indeed, an increase in pH from 7.1 to 8.5 translates into a 35% decrease in the luminescence intensity of the sensitive quantum dots. Thus, our operating principles for luminescence switching can efficiently transduce a chemical stimulation into a change in the emissive response of semiconductor nanoparticles. In principle, this protocol can be extended from hydroxide anions to other target analytes with appropriate adjustments in the molecular design of the chromogenic ligands. It follows that luminescent chemosensors, based on the unique photophysical properties of semiconductor quantum dots, can eventually evolve from our design logic and choice of materials.     

Caged quantum dots

Photoactivatable organic fluorophores and fluorescent proteins have been widely adopted for cellular imaging and have been critical for increasing temporal and spatial resolution, as well as for the development of superresolution microscopy techniques. At the same time, semiconducting nanocrystal quantum dots (QDs) have shown superior brightness and photostability compared to both organic fluorophores and proteins. As part of our efforts to develop nanoparticles with novel optical properties, we have synthesized caged quantum dots, which are nonluminescent under typical microscopic illumination but can be activated with stronger pulses of UV light. We show that ortho-nitrobenzyl groups efficiently quench QDs of different compositions and emissions and can be released from the nanoparticle surface with UV light, both in solution and in live cells. This caging is dependent on the emission of the QD, but it is effective through the visible spectrum into the nIR, offering a large array of new colors for photoactivatable probes. Like organic and protein-based photoactivatable probes, caged QDs can confer increased spatial and temporal resolution, with the added brightness and photostability of QDs.     

Hydrophilic semiconductor quantum dots

Comparative analysis of recent literature data on hydrophilization of semiconductor quantum dots, which are actively used at present in various fields, has been performed. The main methods of preparation of hydrophilic quantum dots are considered: synthesis of the particles in aqueous solutions; replacement of hydrophobic ligands with hydrophilic ligands in the shells stabilizing the particles; creation of a second, water-soluble shell around the hydrophobic particles; and various methods of post-preparative treatment to improve photoluminescent properties of quantum dots.     

Arylthiolate-protected silver quantum dots

This paper describes a new, organic-soluble 4-tert-butylbenzyl mercaptan (BBT) monolayer-protected silver cluster (AgBBT MPC) as the first example of a dissolved silver nanoparticle that exhibits quantized one-electron double layer charging (QDL) voltammetry. Polydisperse AgBBT MPCs made by two different synthetic protocols, but with similar average core diameters (2.1 nm), exhibit sharply differing electrochemistry and optical absorbance spectra. A two-phase procedure (organic/aqueous, termed Prep A-AgBBT) produced MPCs exhibiting a 475 nm surface plasmon absorbance and QDL voltammetry. Neither property was seen for MPCs made by a single-phase procedure, termed Prep B-AgBBT. The difference is thought to reflect poor passivation to oxide formation in the latter Prep B procedure, which is supported by X-ray photoelectron spectroscopy results. Thermogravimetry, mass spectra, and electrochemistry results suggest an average stoichiometric formula of Ag140BBT53, but transmission electron microscopy shows that the products are also polydisperse and include polycrystalline aggregates. Dry, cast films of both Ag MPC preparations on interdigitated array electrodes exhibit low electron hopping conductivity, compared to Au MPCs.     

Size control of mesoscale aqueous assemblies of quantum dots and block copolymers

Dropwise addition of water to blend solutions of block copolymer-stabilized quantum dots (QDs) and amphiphilic block copolymer stabilizing chains PS(665)-b-PAA(68) (PS = polystyrene, PAA = poly(acrylic acid)) in DMF induces self-assembly to form photoluminescent mesoscale QD/block copolymer colloids in water termed QD compound micelles (QDCMs). Here we demonstrate reproducible kinetic control of QDCM particle size and chain stretching within the external PAA stabilizing layer via changes in the initial polymer concentration and rate of water addition. By increasing the initial polymer concentration or decreasing the rate of water addition for a constant blend composition, larger QDCM particles are obtained. From a combination of transmission electron microscopy and dynamic light scattering, the thickness of the external PAA layer is determined for various QDCM sizes, showing that PAA stretching in the external brush layer increases with increasing particle size, reaching the limit of fully extended chains for sufficiently large particles. The photoluminescence spectra from QDCMs in pure water indicate that photoluminescence properties of the block copolymer-stabilized QD building blocks are retained during self-assembly. The demonstrated control of mesoscale particle size and conformation of the stabilizing PAA layer, among other related structural parameters, via simple variation of experimental conditions is a promising step toward the application of QDCM assemblies in photonics and biolabeling.     

Studies on the particle size control of gelatin microspheres

A series of gelatin microspheres (GMs) were prepared through emulsification-coacervation method in water-in-oil (w/o) emulsions. The influence of preparation parameters on particle size, surface morphology, and dispersion of GMs was examined. The studied preparation parameters include concentration of gelatin solutions, concentration of the emulsifier, w/o ratio, emulsifying time, stirring speed, and so on. The surface morphology, dispersion, and particle sizes of GMs were determined by the scanning electron microscopy (SEM), SemAfore 4 Demo software, and particle size distribution graphic charts. The experimental results indicated that increasing the concentration of gelatin solution would increase the particle size of GMs. When the solution concentration increased from 0.050 to 0.200 g/mL gradually, the particle size increased correspondingly. The relationship between the two quantities was linear. On the contrary, increasing the concentration of the emulsifier would decrease the particle size of GMs. Furthermore, the particle size reduced quickly at initial time and slowed down latterly. With the increase of emulsifier concentration from 0 to 0.020 g/mL, themean diameters ofGMsdecreased from 17.32 to 5.38 μm. However, the particle size dwindled slowly when emulsifier concentration was higher than 0.020 g/mL. The excellent result was obtained with the condition of 0.050 g/mL of emulsifier concentration, 0.100 g/mL of gelatin solution concentration, 1/5 of w/o ratio, 10 min of emulsifying time, and 900 r/min of the stirring speed. The GMs prepared at this condition had the smallest sizes, the narrowest size distribution, the best spherical shape, and fluidity. The w/o ratio has the same influence on particle size of GMs as that of gelatin solution concentration. With the increase of w/o ratio, the average particle sizes increased linearly, and the surface of microspheres become smoother as well. It is supposed that w/o ratio can be used to change the diameters and surface morphologies of GMs. The emulsifying time has little influence on the mean diameters of GMs, but it affects the dispersion of GMs apparently. When the emulsifying time was shorter than 5 min, the GMs had bad dispersion. After increasing the emulsifying time to 13 min, the dispersion of GMs changed greatly, whereas the dispersion of GMs became bad again when the emulsifying time was longer than 13 min. According to the experimental results, 13 min was considered to be the best emulsifying time. The stirring speed has the similar influence on GMs’ morphologies as that of emulsifying time. Slow stirring rate made large size distribution and bad spherical shape of GMs; excessive stirring speed results in aggregation among GMs likewise. The smaller size distribution and better spherical shape of GMs were observed under the stirring rate between 500 and 1500 r/min by SEM. In conclusion, increasing the concentration of gelatin solution or w/o ratio would increase the particle sizes of GMs, increasing the concentration of the emulsifier would decrease the sizes of GMs at proper emulsifying time, and stirring speed would get the best spherical shape of GMs. These are the basic laws governing the design and manufacture of the GMs. __________ Translated from Acta Polymerica Sinica, 2008, 8 (in Chinese)