Structure and optoelectronic properties of PbSe quantum dots /PVA. Does the polymer molecular weight matter?

Polyvinyl alcohol (PVA) with different molecular weights (8000, 14,000, and 132,000 g/mol) capped lead selenide (PbSe) quantum dots (QDs) are prepared. The nanocomposites are characterized by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). XRD and TEM studies show that the particle size of PbSe QDs decrease with the increase in PVA molecular weight and/or PVA amount. This may be due to the increase in molecular weight inhibiting further growth of PbSe into the polymer matrix. Thermogravimetric analysis showed that the introduction of PbSe QDs into PVA decreases the crystallinity of the polymer. The optical absorption spectroscopy of prepared nanocomposites showed that the absorption peaks are strongly shifted to the lower wavelength (blue shift) from near infrared region to visible region by increasing the PVA molecular weight. The (I–V) characteristic curves of the PVA/PbSe nanocomposite films under illumination showed a photovoltaic cell‐like behavior. The results indicated that as the molecular weight of polymer increases, the conversion efficiency increases. Copyright © 2010 John Wiley & Sons, Ltd.     

Aspect ratio dependent air stability of PbSe nanorods and photovoltaic applications

Development of unique strategies to overcome Shockley–Queisser (SQ) limit in solar cells has gained a great deal of interest. Multiple exciton generation (MEG) process has been considered as one of the best approaches to the SQ limitation. In this respect, PbSe quantum dots (QDs) and nanorods (NRs) have been regarded as promising solar energy harvesting materials owing to their noticeable MEG yields. Although air stability has been regarded as one of the main disadvantage of PbSe QDs, no study has pointed out to the air sensitivity of PbSe NRs yet. Here, we reveal the effect of aspect ratio on air sensitivity and optical properties of PbSe NRs and discover that NRs with higher aspect ratios are more air stable, attributed to the reduced density of NR ends with air sensitive {100} facets. Furthermore, a band offset was created by utilization of tetrabutylammonium iodide and 1,2-ethanedithiol ligands in cell designs. We found that solar cells based on pristine PbSe NRs are limited by low open circuit voltages due to leakage current pathways. On the other hand, modified cells comprising light absorbing layers prepared by blending NRs and QDs and hole transporting QD layer exhibit a 10-fold improvement in solar cell efficiency.     

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.     

Structural and Size Effects on the Spectroscopic and Redox Properties of CdSe Nanocrystals in Solution: The Role of Defect States

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid‐sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron‐transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle‐in‐a‐box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size‐dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.     

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.     

Ligand-dependent particle size control of PbSe quantum dots

Colloidal solutions of monodisperse PbSe quantum dots (QDs) were synthesized by a hot solution chemical method from a reaction mixture of lead oleate and TOPSe (TOP: tri-n-octylphosphine). The synthesis was carried out at a fixed temperature (170 degrees C) and time, while the particle sizes of the PbSe QDs were controlled by using two different kinds of organic ligands with varied chain length. It was seen that the tuning of PbSe QDs are possible by using the proper molar ratio of the co-ligands, such as acetic acid or hexanoic acid, at a fixed reaction temperature and time, verified by TEM and XRD as well as NIR absorption analysis. The effects of different organic acids were studied and the role of additional organic acids might be due to the extent of ligand exchange efficiency between the Pb oleate and acetic/hexanoic acid in the initial stage, which is caused by the steric hindrance effects of the acids.     

A reverse cation-exchange route to hollow PbSe nanospheres evolving from Se/Ag2Se core/shell colloids

A reverse cation-exchange approach for the synthesis of hollow PbSe nanospheres is successfully established. This route involves a new strategy of a stepwise, in-situ template-based evolution from spherical amorphous Se colloids to Se/Ag(2)Se core/shell colloids, then to hollow PbSe nanospheres. Se colloids are prepared as the initial product by utilizing the chelation of ethylenediamine to bulk Se. They are converted into Se/Ag(2)Se core/shell colloids through the reaction with Ag(+) in ethylene glycol. During the conversion from Ag(2)Se shell to PbSe shell, a small amount of tributylphosphine is crucial as the capping agent. The characterization results, including XRD, SEM, TEM, HRTEM, and EDX, reveal that hollow PbSe nanospheres with polycrystalline and cubic structure are prepared. The corresponding optical band gap is calculated to be 0.56 eV. This conformation is potentially beneficial to the improvement concerning the applications of PbSe nanostructures.     

A photoelectrochemical study of anodic oxides on lead selenide surfaces in alkaline solutions

Lead selenide is a narrow gap semiconductor material. It finds applications in infrared emitting and detecting devices. Their performance is closely related to charge carrier recombination at the surface, which can be reduced by passivation, e.g. due to PbSeO₃ formation by anodic oxidation in alkaline solutions. In dependence on the pretreatment of the surface, two different types of oxide formation were observed. To determine the electronic properties of the anodic oxide, the wavelength dependence of the photocurrent was investigated. The energy of the band gap of both types of anodic oxide on PbSe has been determined to be 2.4 eV for the direct and 1.8 eV for the indirect band gap. A weakening of the photocurrent generated in the bulk (PbSe) due to scattering or absorption within the oxide confirms the potential dependence of the oxide thickness for a high field growth mechanism.     

Facile Fabrication and Optical Properties of Nanostructural PbSe Films

PbSe films with different nanostructures, such as nanoparticles, nanohollows and hierarchical structures, can be synthesized by adjusting the current density and the reaction temperature via a convenient and efficient electrochemical route in the absence of hard template and surfactant. The calculated band gaps of the prepared PbSe nanoparticles and nanohollows were about 0.32 and 0.43 eV, respectively. This suggests that quantum size effect in nanohollows greatly influences their band gap. This preparation method possesses remarkable advantages, such as low cost, high efficiency and easy preparation, which are very suitable for preparing nanomaterials.     

Synthesis and photophysics of red emitting RNA templated PbSe nanostructures

The presence of excess Pb(2+) in the building block consisting of RNA-mediated PbSe QDs induces polarization in PbSe resulting in an increased red fluorescence associated with enhanced charge separation and supramolecular interactions between different building blocks to produce nanotubular morphology in self-assembly.