Lead Oleate Research Articles

Probing Excitons in Ultrathin PbS Nanoplatelets with Enhanced Near-Infrared Emission.

Colloidal PbS nanoplatelets (NPLs) are highly interesting materials for near-infrared optoelectronic applications. We use ultrafast transient optical absorption spectroscopy to study the characteristics and dynamics of photoexcited excitons in ultrathin PbS NPLs with a cubic crystal structure. NPLs are synthesized at near room temperature from lead oleate and thiourea precursors; they show an optical absorption onset at 680 nm (1.8 eV) and photoluminescence at 720 nm (1.7 eV). By postsynthetically treating PbS NPLs with CdCl2, their photoluminescence quantum yield is strongly enhanced from 1.4% to 19.4%. The surface treatment leads to an increased lead to sulfur ratio in the structures and associated reduced nonradiative recombination. Additionally, exciton-phonon interactions in pristine and CdCl2 treated NPLs at frequencies of 1.96 and 2.04 THz are apparent from coherent oscillations in the transient absorption spectra. This study is an important step forward in unraveling and controlling the optical properties of IV-VI semiconductor NPLs.

PbS Nanocrystals Made with Excess PbCl2 Have an Intrinsic Shell that Reduces Their Stokes Shift.

The use of excess PbCl2 in the synthesis of PbS nanocrystals has become a convenient route to produce narrow-line-width infrared emitters. However, these materials have found limited adoption in optoelectronic devices-even compared to PbS nanocrystals prepared with lead oleate. Here, using both transmission electron microscopy and small-angle X-ray scattering, we show that excess PbCl2 results in larger-diameter PbS nanocrystals for the same excitonic features, which is consistent with the formation of an intrinsic insulating shell. We observe further differences in excess-lead-chloride nanocrystals consistent with a shell, including lattice strain and smaller Stokes shifts for intermediate sizes (⌀: 4.8-6.8 nm) that match the passivation/rigidification predicted for a chloride-terminate surface. Our results clarify and rationalize the divergent properties of PbS nanocrystals prepared using different synthetic methodologies, give guidance for device implementation, and offer a new target for synthetic control.

Colloidal Synthesis of Bulk-Bandgap Lead Selenide Nanocrystals.

Lead selenide quantum dots (QDs) are low-bandgap IV-VI semiconducting nanomaterials that have been studied for a variety of applications. Their preparation using colloidal methods can create small spherical to larger cubic nanocrystals, with an upper limit of ~17 nm reported to date. Here we describe methods for preparing cubic PbSe nanocrystals over a 20–40 nm size range using a twostep procedure. Specifically, ~10 nm PbSe QDs are generated using the rapid injection method, the products from which are overcoated with additional lead and selenium precursors. The use of two lead reagents were studied; lead oleate resulted in a maximum of 20 nm cubes, while more reactive lead hexyldecanoate resulted in much larger nanomaterials with bulk bandgaps. However, PbSe samples prepared with lead hexyldecanoate also contained agglomerates. Special care must be taken when characterizing larger strained nanomaterials with X-ray powder diffraction, for which the Scherrer equation is inadequate. A more rigorous approach using the Williamson–Hall method provides characterizations that are consistent with electron microscopy analysis.

Controlled synthesis of brightly fluorescent CH3NH3PbBr3 perovskite nanocrystals employing Pb(C17H33COO)2 as the sole lead source.

Organometal halide perovskite nanocrystals hold vast potential for application in photovoltaics, light emitting diodes, low-threshold lasers, and photodetectors due to their size-tunable bandgap energies and photoluminescence as well as excellent electron and hole mobilities. However, the synthesis of such nanocrystals typically suffers from poor structural stability in solution and the coexistence of lamellate nanocrystals (nanoplatelets) and spherical nanocrystals (nanoparticles). Here we show that the pure nanoparticle morphology of CH3NH3PbBr3 nanocrystals can be realized by employing lead oleate (Pb(C17H33COO)2) as the sole lead source and controlled using short- and long-chain mixed alkyl ammonium. These nanocrystals are monodispersed (2.2 ± 0.4 nm in diameter), highly fluorescent (with a quantum yield approaching 85%), and highly stable in the solution (for more than 30 days). Comparative studies reveal that the shape of CH3NH3PbBr3 nanocrystals is strongly dependent on the lead source, PbBr2 and Pb(C17H33COO)2, and evolves as a function of the ratio of short- and long-chain alkyl ammoniums in the precursors. At an optimal short to long-chain alkyl ammonium ratio of 4 : 6, the growth of CH3NH3PbBr3 nanoplatelets can be selectively suppressed with Pb(C17H33COO)2 as the sole lead source, enhancing the overall photoluminescence quantum yield of the produced CH3NH3PbBr3 nanocrystals. This work reveals important new insights for controlled synthesis of perovskite nanocrystals with pure crystal shape and significantly improved photoluminescence properties and stability.

A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit.

We report a tunable library of N,N,N'-trisubstituted selenourea precursors and their reaction with lead oleate at 60-150 °C to form carboxylate-terminated PbSe nanocrystals in quantitative yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6-56.7 μM) and the size following complete precursor conversion (d = 1.7-6.6 nm) to be controlled. Narrow size distributions (σ = 0.5-2%) are obtained whose spectral line widths are dominated (73-83%) by the intrinsic single particle spectral broadening, as observed using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320-65 meV, d = 1.6-4.4 nm) that derives from exciton fine structure and exciton-phonon coupling rather than broadening caused by the size distribution.

Effects of lead ions on the flotation of hemimorphite using sodium oleate

The effects of Pb(II) ions on the flotation of hemimorphite was investigated by micro-flotation tests, zeta-potential measurements, solution chemistry and Fourier transform infrared spectroscopy. The micro-flotation results indicated that the Pb(II) effectively improved the flotation of hemimorphite minerals. Good floatability of minerals was obtained in the pH region 7 to 9. Adsorption/precipitation of the hydrolyzed species of lead cations occurred in this pH region, these species promote sodium oleate adsorption and from lead oleate on the surface of hemimorphite.

Simple Molecular Reactive Force Field for Metal-Organic Synthesis.

For colloidal quantum dots to transition from research laboratories to deployment as optical and electronic products, there will be a need to scale-up their production to large-scale manufacturing processes. This demand increases the need to understand their formation via a molecular representation of the nucleation of lead sulfide (PbS) quantum dot systems passivated by lead oleate complexes. We demonstrate the effectiveness of a new type of reactive potential, custom-made for this system, that is drawn from simple Morse, Lennard-Jones, and Coulombic components, which can reproduce reactions across a broad range of PbS quantum dot sizes with good accuracy. We validate the capability of this model to capture reactive systems by comparison to ab initio calculations for a reaction between two dots.

Size-controlled Synthesis and Magnetic Properties of FePt/PbS Core/Shell Nanoparticles

A method to prepare FePt/PbS core/shell nanoparticles with a controllable PbS thickness (dPbS) based on the reaction of lead oleate and sulfur in the presence of FePt nanoparticles has been develop...

Formation of Ultrasmall PbS Nanocrystals in Octadecene at Mild Temperature Promoted by Alcohol or Acetone Injection

The formation of ultrasmall PbS nanoparticles by the reaction of lead oleate with bis(trimethylsilyl) sulfide in octadecene at a temperature as low as 50 °C can be strongly promoted by addition of protic solvents (alcohols, water) or acetone. The proposed mechanism involves formation of the Pb(oleate)2-(TMS)2S complex which upon the injection of a proper solvent forms highly reactive Pb- and S-containing intermediates. The different reactive ability of these intermediates depending on the type of promoter allows one to prepare PbS QDs with different size. Such a mild-temperature process is useful for production of ultrasmall monodisperse PbS colloidal quantum dots due to suppressed recrystallization of PbS nanoparticles.

Role of Organosulfur Compounds in the Growth and Final Surface Chemistry of PbS Quantum Dots

This paper describes the mechanism by which reaction of sulfur with 1-octadecene (ODE) induces a change in the shape of PbS quantum dots (QDs), synthesized from the S/ODE precursor and lead(II) oleate, from cubic to hexapodal by altering the ligand chemistry of the growing QDs. 1H NMR and optical spectroscopies indicate that extended heating of sulfur and ODE at 180 °C produces a series of organosulfur compounds with optical transitions in the visible region and that the binding of organosulfur ligands to the growing QD induces a preferential growth at the ⟨100⟩ faces (over the ⟨111⟩ faces) and, therefore, a hexapodal geometry for the particles. The study shows that S/ODE can be made a more reliable precursor by reducing the temperature and duration of the sulfur dissolution step and that any metal sulfide QD synthesis using elemental sulfur heated to high temperatures should take steps to reduce the in situ yield of organosulfur byproducts by avoiding olefinic solvents.

The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots

A combination of FT-IR, (1)H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chemical shifts of their protons, and their diffusion coefficients. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb(2+) ion or bridges two Pb(2+) ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is approximately 1:1 for all sizes of the QDs we studied. The second oleate species in these samples bridges two Pb(2+) ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ~1.4 nm), which are byproducts of the QD synthesis. The concentration of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concentration of the oleic acid ligand in the reaction mixture. The oleate molecules on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Additional washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quantitatively characterizes the distribution of binding geometries at the inorganic/organic interface of the nanocrystals and demonstrates the utility of using organic ligands as probes for the composition of a colloidal QD sample as a function of the preparation procedure.

On the role of Pb0 atoms on the nucleation and growth of PbSe and PbTe nanoparticles

In this contribution, a nucleation and growth mechanism of PbSe and PbTe nanoparticles are proposed. The formation and growth of PbSe and PbTe nanoparticles during their reaction synthesis were studied and followed using transmission electron microscopy, and their related techniques. In the synthesis method, trioctylphosphine-selenide and telluride were used as the chalcogen precursors, while lead oleate was employed as the lead precursor. Different synthesis conditions were tested to assess the effect of varying the reaction time, lead to chalcogen ratio, reaction temperature, and lead oleate concentration. The synthesized nanoparticles were characterized by means of electron diffraction, energy dispersive X-ray spectroscopy, scanning transmission electron microscopy, and electron energy loss spectroscopy, to obtain information related to their morphology, crystal structure, and composition. The experimental results suggest that the growth of the lead chalcogenide nanoparticles greatly relies on the reduction of Pb2+ ions to Pb0 atoms at early reaction times; this reduction of the lead precursor is evidenced by the formation of Pb nanoparticles with sizes between 1 and 3 nm under certain synthesis conditions. These Pb nanoparticles gradually disappear as the reaction progresses, suggesting that the reduced Pb0 atoms are able to contribute to the growth of the PbSe and PbTe nanoparticles, reaching sizes between 8 and 18 nm. The current results contribute to a better understanding of the nucleation and growth mechanisms of lead chalcogenide nanoparticles, which will enable the definition of more efficient synthesis routes of these types of nanostructures.

Electrochemical characterization of PbS quantum dots capped with oleic acid and PbS thin films — a comparative study

This research aims at probing electrochemical response of oleic acid capped PbS quantum dots deposited on a Pt electrode in 0.1 M aqueous sodium hydroxide solution by cyclic voltammetry and chronoamperometry. Quantum dots were also characterized by photoluminescence and IR spectroscopy. Cyclic voltammetry of bulk PbS thin films obtained via chemical bath deposition is investigated in order to interpret the data on PbS quantum dots. It was found that the two materials exhibit essentially similar voltammetric behavior; however, oxidation of PbS quantum dots tends to start at slightly more cathodic potentials than that of bulk PbS films. This effect is attributed to the influence of capping oleic acid that binds lead ions into an insoluble lead oleate thereby causing the cathodic shift of the formal redox potential. The procedures designed to partially remove oleic acid from PbS quantum dots result in the anodic shift of the PbS quantum dots oxidation towards the values characteristic for the bulk material. A possibility of determining absolute positions of conduction and valence bands in quantum dots by cyclic voltammetry is discussed but the influence of the energy level structure of PbS quantum dots on their voltammetric response was not revealed under the conditions of the present study. However, PbS quantum dots can withstand multiple redox cycles whereas bulk PbS films dissolve readily upon the first oxidation. The effect was attributed to the oleic acid layers on the PbS quantum dots surface, those layers preventing soluble oxidation products from diffusing into the bulk of solution. Certain PbS quantum dots samples showed remarkable stability against oxidation typical for PbS and starting at −0.2 V vs Ag/AgCl (sat.) and a stable response at oxidation and reduction at higher (0.55 V) and lower (−0.8 V) potentials, respectively.

Predicting Nanocrystal Shape through Consideration of Surface-Ligand Interactions

Density functional calculations for the binding energy of oleic acid-based ligands on Pb-rich {100} and {111} facets of PbSe nanocrystals determine the surface energies as a function of ligand coverage. Oleic acid is expected to bind to the nanocrystal surface in the form of lead oleate. The Wulff construction predicts the thermodynamic equilibrium shape of the PbSe nanocrystals. The equilibrium shape is a function of the ligand surface coverage, which can be controlled by changing the concentration of oleic acid during synthesis. The different binding energy of the ligand on the {100} and {111} facets results in different equilibrium ligand coverages on the facets, and a transition in the equilibrium shape from octahedral to cubic is predicted when increasing the ligand concentration during synthesis.

In‐Situ Observation of Nucleation and Growth of PbSe Magic‐Sized Nanoclusters and Regular Nanocrystals

In-situ observation of the temporal evolution of the absorption of PbSe nanocrystals (NCs) via a low-temperature noninjection approach is presented. Based on a model reaction of lead oleate (Pb(OA)(2) ) and n-trioctylphosphine selenide (TOPSe) in 1-octadecene at 35-80 °C, the use of commercially available TOP (90 or 97%) in affecting the formation of the NCs is explored. TOPSe solutions made from TOP 90% exhibited higher reactivity than those made from TOP 97%. (31)P NMR spectroscopy detected no dioctylphosphine selenide (DOPSe) but some DOP in ≈1.0 M TOPSe/TOP solution (made from TOP 90%), as well as no diphenylphosphine selenide (DPPSe) when DPP was added to the ≈1.0 M solution. Hence, it is proposed that, for the formation of PbSe monomers, an indirect pathway dominates with the formation of a Pb-P complex/intermediate, which results from the activation of Pb(OA)(2) by a phosphine compound (such as DPP, DOP, or TOP) and in turn reacts with TOPSe. With the use of TOP 90% and the addition of secondary phosphine DPP, the formation of PbSe magic-sized nanoclusters (MSNCs) and regular NCs (RNCs) is investigated. With proper tuning of the synthesis conditions, the formation of various PbSe MSNCs versus RNCs is monitored in situ with versus without the addition of DPP, or at different reaction temperatures but otherwise identical synthetic formulation and reaction parameters. Accordingly, the degree of supersaturation (DS) of the PbSe monomer affecting the development of these PbSe MSNCs versus RNCs is proposed; the higher the DS, the more the MSNCs are favored. Also, surface-determined cluster-cluster aggregation is proposed to be the growth mechanism for both the RNCs and MSNCs. For the former, quantized growth is followed by continuous growth. For the latter, the sizes of the magic-sized families are calculated.

Two‐Phase Approach to High‐Quality, Oil‐Soluble, Near‐Infrared‐Emitting PbS Quantum Dots by Using Various Water‐Soluble Anion Precursors

AbstractColloidal PbS quantum dots (QDs) with tunable photoemission throughout the near‐infrared (NIR) region (ca. 750–1000 nm) were synthesized by a two‐phase approach. Here, oil‐soluble lead oleate formed by a reaction of lead acetate and oleic acid (OA, the capping agent) in n‐decane at 130 °C was used as lead precursor, and water‐soluble Na2S, thioacetamide (TAA), and thiourea, each having a different reactivity, were used as sulfur sources. When an n‐decane solution of lead precursor and an aqueous solution of sulfur precursor were mixed at the appointed temperature, oil‐soluble, near‐infrared‐emitting PbS QDs were achieved. In this study, we investigated the influence of the reactivity of water‐soluble sulfur sources on the synthesis of PbS QDs. The morphology and crystal structure of the as‐prepared PbS QDs were characterized by (high‐resolution) transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X‐ray diffraction (XRD).

Synthesis of Monodisperse Superconducting Lead Nanocrystals

We report on a new organic-phase synthesis method to produce a large quantity of high-quality monodisperse population of superconducting lead (Pb) nanocrystals. They are obtained from the alcohol reduction of a mixture of lead oleate and lead acetate in the presence of trioctylphosphine in octadecene. We show that the details of lead carboxylate mixture preparation is the key parameter in controlling the nucleation rate of the synthesis. Because mixtures of metal carboxylate have been widely used in synthesizing various high-quality metals or metal−oxide nanocrystals, the results herein should contribute to the understanding of the detailed mechanisms of these syntheses. Because of the large quantities provided by this synthesis, these nanocrystals are particularly well adapted for studies of the effect of quantum confinement on superconductivity.

Surface Chemistry of Colloidal PbSe Nanocrystals

Solution nuclear magnetic resonance spectroscopy (NMR) is used to identify and quantify the organic capping of colloidal PbSe nanocrystals (Q-PbSe). We find that the capping consists primarily of tightly bound oleic acid ligands. Only a minor part of the ligand shell (0-5% with respect to the number of oleic acid ligands) is composed of tri- n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concentration. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and organic ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidation leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.

Synthesis and Characterization of PbSe Nanocrystals by a Microchannel Reactor

The synthesis of high-quality monodispersed nanocrystal is very important. Typical synthetic method is rapid nucleation by injection of an organometallic precursor into a solvent maintaining the reaction temperature. Since these methods are discontinuous processes, they are not efficient for large-scale production of monodisperse nanocrystals. In this study, continuous microchannel reaction technique is presented for synthesis of monodisperse lead selenide nanocrystals in a diphenyl ether as high-temperature organic media. The microchannel reactor was used due to its possibility of continuous process and reproducibility of narrow size distribution in nanocrystal synthesis. The synthesis was carried out in microchannel reactor (800 μm diameter) made from PTFE. Lead oleate and TOP-Se were used as organic precursor and diphenyl ether as high-temperature organic solvents. Lead selenide particles with a size of less than 10nm could be continuously prepared by this method. The nanocrystals have been characterized by X-ray diffraction, TEM and optical absorption spectrometer.

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 °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.