Low-temperature approach to high-yield and reproducible syntheses of high-quality small-sized PbSe colloidal nanocrystals for photovoltaic applications

Small-sized PbSe nanocrystals (NCs) were synthesized at low temperature such as 50-80 °C with high reaction yield (up to 100%), high quality, and high synthetic reproducibility, via a noninjection-based one-pot approach. These small-sized PbSe NCs with their first excitonic absorption in wavelength shorter than 1200 nm (corresponding to size < ～3.7 nm) were developed for photovoltaic applications requiring a large quantity of materials. These colloidal PbSe NCs, also called quantum dots, are high-quality, in terms of narrow size distribution with a typical standard deviation of ～7-9%, excellent optical properties with high quantum yield of ～50-90% and small full width at half-maximum of ～130-150 nm of their band-gap photoemission peaks, and high storage stability. Our synthetic design aimed at promotion of the formation of PbSe monomers for fast and sizable nucleation with the presence of a large number of nuclei at low temperature. For formation of the PbSe monomer, our low-temperature approach suggests the existence of two pathways of Pb-Se (route a) and Pb-P (route b) complexes. Either pathway may dominate, depending on the method used and its experimental conditions. Experimentally, a reducing/nucleation agent, diphenylphosphine, was added to enhance route b. The present study addresses two challenging issues in the NC community, the monomer formation mechanism and the reproducible syntheses of small-sized NCs with high yield and high quality and large-scale capability, bringing insight to the fundamental understanding of optimization of the NC yield and quality via control of the precursor complex reactivity and thus nucleation/growth. Such advances in colloidal science should, in turn, promote the development of next-generation low-cost and high-efficiency solar cells. Schottky-type solar cells using our PbSe NCs as the active material have achieved the highest power conversion efficiency of 2.82%, in comparison with the same type of solar cells using other PbSe NCs, under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm(2).     

Arrays of ZnO/Zn(x)Cd(1-x)Se nanocables: band gap engineering and photovoltaic applications

Arrays of ZnO/Zn(x)Cd(1-x)Se (0 ≤ x ≤ 1) core/shell nanocables with shells of tunable compositions have been synthesized on fluorine-doped tin oxide glass substrates via a simple ion-exchange approach. Through the effects of stoichiometry and type II heterojunction, optical absorptions of the nanocable arrays can be controllably tuned to cover almost the entire visible spectrum. Lattice parameters and band gaps of the ternary Zn(x)Cd(1-x)Se shells were found to have respectively linear and quadratic relationships with the Zn content (x). These ZnO/Zn(x)Cd(1-x)Se nanocable arrays are further demonstrated to be promising photoelectrodes for photoelectrochemical solar cells, giving a maximum power conversion efficiency up to 4.74%.     

Low-temperature synthesis and characterization of Ag2S1−x Te x (0≤x≤1)

A low-temperature route for the synthesis of Ag₂S, Ag₂Te and their solid solutions Ag₂S_{1 −x} Te _x (0 ≤x ≤ 1) is reported. Ag₂S is prepared by the direct addition of silver nitrate solution to thiourea, while Ag₂ Te is prepared by reacting silver nitrate solution with tellurium in nitric acid and subsequently reducing it with hydrazine hydrate. The solid solutions of Ag₂S and Ag₂Te are obtained by the addition of nitrate solutions of silver and tellurium to thiourea followed by its reduction with hydrazine hydrate. The method enables the synthesis of low-temperature crystalline phase of Ag₂S_{1 −x} Te _x solid solutions. The powder X-ray diffraction studies suggest that the solid solutions of compositionsx 〈 0·3 have a phase akin toα-Ag₂S and those with compositionsx 〉 0·6 are similar toα-Ag₂ Te. In the intermediate range of compositions (x=0·4 and 0·5), the solid solutions are found to be mixtures ofα-Ag₂S andα-Ag₂ Te phases which transform totally toα-Ag₂S phase on prolonged annealing at about 473 K.     

Hybrid nanocrystal/polymer solar cells based on tetrapod-shaped CdSe(x)Te(1-x) nanocrystals

A series of ternary tetrapodal nanocrystals of CdSe(x)Te(1-x) with x = 0?(CdTe), 0.23, 0.53, 0.78, 1 (CdSe) were synthesized and used to fabricate hybrid nanocrystal/polymer solar cells. Herein, the nanocrystals acted as electron acceptors, and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) was used as an electron donor. It was found that the open circuit voltage (V(oc)), short-circuit current (J(sc)) and power conversion efficiency (η) of the devices all increased with increasing Se content in the CdSe(x)Te(1-x) nanocrystals under identical experimental conditions. The solar cell based on the blend of tetrapodal CdSe nanocrystals and MEH-PPV (9:1?w/w) showed the highest power conversion efficiency of 1.13% under AM 1.5, 80?mW?cm(-2), and the maximum incident photon to converted current efficiency (IPCE) of the device reached 47% at 510?nm. The influence of nanocrystal composition on the photovoltaic properties of the hybrid solar cells was explained by the difference of the band level positions between MEH-PPV and the nanocrystals.     

Birefringence of Zn(x)Cd(1-x)S near the isotropic point

The birefringence of the mixed crystal Zn0.14Cd0.86S has been measured near its isotropic point and found to be highly dispersive. The region of strong dispersion shifts to shorter wavelengths than observed in pure CdS, and its magnitude near the isotropic point is larger. These results are highly significant for the development of narrowband dispersive birefringent filters in the blue-green region of the spectrum.     

Photovoltaic devices employing ternary PbS_xTe_(1-x) nanocrystals

Colloidal quantum dots(CQDs), especially lead chalcogenide CQDs, are regarded as promising materials for the next generation solar cells, due to their large absorption coefficient, excellent charge transport,and multiple exciton generation effect. We successfully synthesized highly-crystalline, monodispersed,well-alloyed PbS_xTe_(1-x) nanocrystals via a one-pot, hot injection reaction method. Energy-filtered transmission electron microscopy suggested that the S and Te anions were uniformly distributed in the alloy nanoparticles. The photovoltaic performance of CQD solar cells based on ternary PbS_xTe_(1-x) was reported for the first time. The photovoltaic devices using PbS_xTe_(1-x) were more efficient than either the pure PbS or the PbTe based devices. In addition, the PbS_xTe_(1-x) based devices showed a significantly improved stability than that of the PbTe based devices.     

Synthesis and magnetic characterizations of La(1-x)Sr(x)MnO3 nanoparticles for biomedical applications

The La(1-x)Sr(x)MnO3 (LSMO) nanoparticles have been synthesized by citric gel process followed by ball milling method. These nanoparticles demonstrated high crystalline quality. Nanoparticle size was further decreased by ball milling technique as observed by the field-emission scanning electron microscopic studies. The ball milled and silica coated LSMO nanoparticles show magnetic transition at about 370 K with a superparamagnetic properties. The ferromagnetic resonance (FMR) spectra analysis of LSMO nanoparticles shows large FMR linewidth due to the surface strain of the nanoparticles. Both magnetization and FMR studies demonstrate that the LSMO nanoparticles are highly anisotropic. The toxicity of the nanoparticles was studied for safe biomedical applications. Measurement of intracellular reactive oxygen species (ROS) and MTT assay results show that LSMO nanoparticles are relatively nontoxic and the toxicity is further reduced by SiO2 coating. These results are very important for applications in the field of biotechnology.     

Control of luminescence emitted by Cd 1-x Mn x S nanocrystals in a glass matrix: x concentration and thermal annealing

Cd(1-x)Mn(x)S nanocrystals (NCs) were successfully grown in a glass matrix and investigated by photoluminescence (PL), electron paramagnetic resonance (EPR) and magnetic force microscopy (MFM). We verified that the luminescent properties of these NCs can be controlled both by changing the x concentration and by thermal annealing of the samples. The EPR and PL data showed that the characteristic emission of Mn(2+) ions ((4)T(1)-(6)A(1)) is only observed when this magnetic impurity is substitutionally incorporated in the Cd(1-x)Mn(x)S NC core (site S(I)). Besides, it was observed that the emission ((4)T(1)-(6)A(1)) suppression, caused by the Mn(2+) ion presence near the surface (site S(II)) of the Cd(1-x)Mn(x)S NCs, is independent of the host material. The MFM images also confirmed the high quality of the Cd(1 - x)Mn(x)S NC samples, showing a uniform distribution of total magnetic moments in the nanoparticles.     

Co(1-x)Mg(x)MoO4 compounds for pressure indicators

Two Co(1-x)Mg(x)MoO(4) oxide compositions (x=0 and x=0.4) were investigated as potential pressure indicators. The first order phase half-transition induced by pressure application from the β to the α form, i.e. from the high temperature/low pressure form to the high pressure/low temperature form, was studied thanks to the powder diffuse reflection (color) evolution versus the applied pressure. Three key parameters were analyzed: (i) the magnesium content, (ii) the powder grain sizes, (iii) the pressure application mode (uniaxial or isostatical). It was shown these three parameters allow tuning the transition pressure in a wide range from few bars to few kbars.     

Synthesis and raman scattering from Zn(1-x)Mn(x)S diluted magnetic semiconductor nanowires

The growth of Diluted Magnetic Semiconducting (DMS) Zn(1-x)Mn(x)S (0 < or = x < 0.6) nanowires (NWs) using a three-zone furnace and two solid sources is reported. The approach is generally applicable to many binary and ternary NW systems that grow by the Vapor-Liquid-Solid growth mechanism. Mn concentration was controlled by the temperature of the Mn source. The Zn/Mn ratio was found to determine the crystalline structure, i.e., wurtzite or zinc blende. High-resolution transmission electron microscopy measurements revealed highly crystalline single phase NWs. The vibrational properties of the DMS NWs with different Zn/Mn ratios were studied by correlating their Raman scattering spectra with the composition measured by Energy Dispersive X-Ray Spectroscopy (EDS). We find that the transverse optical (TO) phonon band disappears at the lowest Mn concentrations, while the longitudinal optical (LO) phonon band position was found insensitive to x. Three additional Raman bands were observed between the ZnS q = 0 TO and LO phonons when Mn atoms were present in the NWs. These bands are similar to those reported previously for bulk Zn(1-x)Mn(x)S and their origin is still controversial.     

Structural,optical and electrical properties of silicon nanocrystals embedded in SixC1−x/SiC multilayer systems for photovoltaic applications

In this work we present a structural, optical and electrical characterization of Si_xC_1?x/SiC multilayer systems with different silicon content. After the deposition process, an annealing treatment was carried out in order to induce the silicon nanocrystals formation. By means of energy-filtered transmission electron microscopy (EFTEM) we observed the structural morphology of the multilayers and the presence of crystallized silicon nanoprecipitates for samples annealed up to 1100 °C. We discuss the suitability of optical techniques such as Raman scattering and reflectance and transmittance (R&T) for the evaluation of the crystalline fraction of our samples at different silicon excess ranges. In addition, the combination of R&T measurements with simulation has proved to be a useful instrument to confirm the structural properties observed by EFTEM. Finally, we explore the origin of the extremely high current density revealed by electrical measurements, probably due to the presence of an undesired defective SiC_yO_z ternary compound layer, already supported by the structural and optical results. Nevertheless, the variation of the electrical measurements with the silicon amount indicates a small but significant contribution from the multilayers.     

Magnetic properties of Cd(1 - x)Mn(x)Te/C nanocrystals

Mn doped CdTe nanocrystals coated by carbon (Cd(1 - x)Mn(x)Te/C) were synthesized by a one-step, kinetically controlled solid state reaction under autogenic pressure at elevated temperatures. Electron microscopic analysis confirmed that the 40-52 nm Cd(1 - x)Mn(x)Te core was encapsulated by a 6-9 nm carbon shell. The efficient doping by Mn(2+) in the zinc blende Cd(1 - x)Mn(x)Te lattice, up to an atomic ratio of Mn/Cd of 0.031, was confirmed from electron paramagnetic resonance (EPR) experiments. In the case of higher doping, it is likely that manganese is partially expelled to the nanocrystal surface. All the doped samples exhibit ferromagnetism at room temperature. The lowest doped sample has the highest magnetic moment (1.91 ± 0.02 μ(B)/Mn). The more concentrated samples exhibit weaker ferromagnetic interactions, probably due to an incomplete coupling between carriers in the host CdTe semiconductor and dopant spins.     

Fabrication, structural characterization and photoluminescence of single-crystal Zn(x)Cd(1-x)S zigzag nanowires

Large-scale synthesis of ternary Zn(x)Cd(1-x)S zigzag nanowires was achieved in a one-step metal-organic chemical vapour deposition (MOCVD) process with co-fed single precursors of ZnS and CdS. Their morphologies, structures and optical properties were characterized and confirmed by scanning electron microscopy, high-resolution transmission electron microscopy, x-ray spectroscopy, and photoluminescence. The Zn(x)Cd(1-x)S zigzag nanowires are single crystalline, with axis [001], by changing the growth direction from [Formula: see text] to [Formula: see text]. Regarding the formation of zigzag nanowires, we suggest that the shear strain and slight fluctuation of the reaction conditions may be the major factors that make the nanowires change growth direction. In addition, because of the lower temperature and versatility, this new fabrication method might present a new and facile way to form other ternary nanomaterials. Furthermore, the green emission of the nanowires may have potential applications in electronic/optical nanodevices.     

Synthesis of Zn(1-x)Cd(x)S:Mn/ZnS quantum dots and their application to light-emitting diodes

3.6?nm sized Mn-doped Zn(1-x)Cd(x)S quantum dots (QDs) with the composition (x) of 1, 0.5, 0.2 and 0 were synthesized by a reverse micelle approach. The bandgap energy of Zn(1-x)Cd(x)S:Mn QDs was tuned to a higher energy by increasing the Zn content, and the actual composition of alloyed Zn(1-x)Cd(x)S:Mn QDs was found to be different from the solution composition. Consecutive overcoating of the Zn(1-x)Cd(x)S:Mn QD surface by a ZnS shell was done, and the core/shell structured QDs exhibited quantum yields of 14-30%, depending on the composition of the core QDs. Using CdS:Mn/ZnS QDs, orange and white light-emitting diodes (LEDs) pumped by a near-UV and blue LED chips, respectively, were fabricated and their optical properties are described.     

Particle-assisted Ga(x)In(1-x)P nanowire growth for designed bandgap structures

Non-tapered vertically straight Ga(x)In(1-x)P nanowires were grown in a compositional range from Ga(0.2)In(0.8)P to pure GaP in particle-assisted mode by controlling the trimethylindium, trimethylgallium and hydrogen chloride flows in metal-organic vapor phase epitaxy. X-ray energy dispersive spectroscopy in transmission electron microscopy revealed homogeneous radial material composition in single nanowires, whereas variations in the material composition were found along the nanowires. High-resolution x-ray diffraction indicates a variation of the material composition on the order of about 19% measuring an entire sample area, i.e., including edge effects during growth. The non-capped nanowires emit room temperature photoluminescence strongly in the energy range of 1.43-2.16 eV, correlated with the bandgap expected from the material composition.     

Nanocrystals of PbSe core, PbSe/PbS, and PbSe/PbSxS(1-x) core/shell as saturable absorbers in passively Q-switched near-infrared lasers

The saturable optical absorption properties of PbSe core nanocrystals (NCs), and their corresponding PbSe/PbScore/shell and PbSe/PbSexS(1-x) core/alloyed-shell NCs, were examined at lambda = 1.54 microm. Saturation intensities of approximately 100 MW/cm2 were obtained. The NCs act as passive Q switches in near-infrared pulsed lasers. Q-switched output pulse energies up to 3 mJ, with a pulse duration of 40-55 ns were demonstrated. Analysis of the optical transmission versus pulse light intensity was carried out according to a model that includes ground-state as well as excited-state absorption. For pulses approximately 10 ns long, the NCs act as fast saturable absorbers. The theoretical fits yield a ground-state absorption cross section of 10-16-10-15 cm2, an excited-state absorption cross section of sigma(es) is congruent to 10(-16) cm2, and an effective lifetime of tau(eff) is congruent to 5 x 10(-12) s.     

Constructing anisotropic single-Dirac-cones in Bi(1-x)Sb(x) thin films

The electronic band structures of Bi(1-x)Sb(x) thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness, and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi(1-x)Sb(x) thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band gap, which can be used in a general two-dimensional system that has a nonparabolic dispersion relation as in the Bi(1-x)Sb(x) thin film system.     

Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

Widely tunable emissions of colloidal Zn(x)Cd(1-x)Se alloy quantum dots using a constant Zn/Cd precursor ratio

The widely tunable emissions of Zn(x)Cd(1-x)Se alloy quantum dots (QDs), which emit green to red wavelengths from 534 to 620 nm, are reported. Green-, yellow-, orange-, and red-emitting QDs were synthesized by varying a point of time for oleylamine (as a co-surfactant) addition and a Se precursor amount, and keeping a constant Zn/Cd precursor ratio. With reaction time the alloying and particle growth of the alloy QDs progressed simultaneously in the opposite direction in the variation of their band gap. However, the band gap energies of all QDs were observed to be gradually blue-shifted due to the slight dominance of alloying over the particle growth effect. The compositions of alloy QDs were estimated based on their sizes and band gap energies. Zn(x)Cd(1-x)Se core QDs were also overcoated with a ZnSe shell with a higher band gap to enhance their quantum yields.