Auger Recombination and Carrier-Lattice Thermalization in Semiconductor Quantum Dots under Intense Excitation.

A thorough understanding of the photocarrier relaxation dynamics in semiconductor quantum dots (QDs) is essential to optimize their device performance. However, resolving hot carrier kinetics under high excitation conditions with multiple excitons per dot is challenging because it convolutes several ultrafast processes, including Auger recombination, carrier-phonon scattering, and phonon thermalization. Here, we report a systematic study of the lattice dynamics induced by intense photoexcitation in PbSe QDs. By probing the dynamics from the lattice perspective using ultrafast electron diffraction together with modeling the correlated processes collectively, we can differentiate their roles in photocarrier relaxation. The results reveal that the observed lattice heating time scale is longer than that of carrier intraband relaxation obtained previously using transient optical spectroscopy. Moreover, we find that Auger recombination efficiently annihilates excitons and speeds up lattice heating. This work can be readily extended to other semiconductor QDs systems with varying dot sizes.

Rapid In Situ Ligand-Exchange Process Used to Prepare 3D PbSe Nanocrystal Superlattice Infrared Photodetectors.

Colloidal semiconductor nanocrystals are important building blocks for low-cost, solution-processed electronic devices with tunable functionalities. Considerable progress is made in improving charge transport through nanocrystal films by exchanging long insulating ligands with shorter passivating ligands. To take full advantage of this strategy, it is equally important to fabricate close-packed structures that reduce the average interparticle spacing. Yet it remains a challenge to retain long-range, close-packed order after ligand exchange. Here, a novel one-step in situ ligand-exchange method is demonstrated that enables rapid (5 min) ligand exchange of nanocrystal films, which are more than 50 layers thick. Using this simple and efficient method, it is shown that the face-centered cubic ordering of 500 nm thick PbSe nanocrystal films is retained after ligand exchange from oleic acid to benzoic acid. Moreover, it is demonstrated that PbSe nanocrystal photodetectors with a well-ordered structure have superior optoelectronic properties compared to disordered films; ordered films have a 16× higher responsivity of ≈0.25 A W-1 at 1 V and a 2× faster response time. As far as it is known, this is the first report to realize a rapid one-step ligand exchange through a thick superlattice film with retention of long-range order.

Miniaturised Infrared Spectrophotometer for Low Power Consumption Multi-Gas Sensing.

Concept, design and practical implementation of a miniaturized spectrophotometer, utilized as a mid-infrared-based multi gas sensor is described. The sensor covers an infrared absorption wavelength range of 2.9 to 4.8 um, providing detection capabilities for carbon dioxide, carbon monoxide, nitrous oxide, sulphur dioxide, ammonia and methane. A lead selenide photo-detector array and customized MEMS-based micro-hotplate are used as the detector and broadband infrared source, respectively. The spectrophotometer optics are based on an injection moulded Schwarzschild configuration incorporating optical pass band filters for the spectral discrimination. This work explores the effects of using both fixed-line pass band and linear variable optical filters. We report the effectiveness of this low-power-consumption miniaturized spectrophotometer as a stand-alone single and multi-gas sensor, usage of a distinct reference channel during gas measurements, development of ideal optical filters and spectral control of the source and detector. Results also demonstrate the use of short-time pulsed inputs as an effective and efficient way of operating the sensor in a low-power-consumption mode. We describe performance of the spectrometer as a multi-gas sensor, optimizing individual component performances, power consumption, temperature sensitivity and gas properties using modelling and customized experimental procedures.

An Evaluation Research About Effects of Characterized Cadmium Selenide (CdSe) and Lead Selenide (PbSe) Quantum Dots on Brine Shrimp (Artemia salina).

Quantum dots (QDs), such as cadmium selenide (CdSe) and lead selenide (PbSe) exhibit excellent optical, magnetic and chemical properties due to their extremely size (ca. 1-10 nm) and are attractive semiconductor nanomaterials for optical studies and energy storage. In this study, aqueous synthesis of CdSe and PbSe QDs in a size range of 2-10 nm was described. Synthesized QDs were characterized using SEM and TEM, DLS, zeta potential, FTIR, EDX and XRD. Highest accumulation (72.5 ± 5.8 mg L-1) of PbSe QDs occurred at 10 ppm suspensions. In general accumulation increased up to 48 h exposure then fluctuate tended to decline. For CdSe QDs, accumulation tended to decrease for 72 h exposure except that for 5 ppm groups. For the elimination period, in general, the elimination levels of PbSe and CdSe QDs from exposed individuals decreased (p < 0.05) even it has some fluctuate.

Silica encapsulation of thiol-stabilized lead selenide (PbSe) quantum dots in aqueous solution.

Silica encapsulation of lead selenide quantum dots (PbSe QDs) in aqueous solution is reported. Thioglycolic acid (TGA) stabilized PbSe QDs were modified with 3-mercaptopropyl trimethoxysilane (MPS) through vigorous stirring in water for 18-24 h in alkaline solution (pH 10.4-10.6). Silica shell was developed by controlled deposition and precipitation of silicates from sodium silicate solution onto MPS modified QDs surfaces. TEM images showed multiple PbSe QDs encapsulated in silica shell. The size of PbSe-SiO2 core-shell nanocrystals was estimated to be 25-30 nm by TEM. Elemental compositions (Pb, Se and Si) were investigated by EDX analysis. The purified colloids of PbSe-SiO2 QDs were stable for months when kept at 4 °C.

Nanostructured PbSe Films Deposited by Spray Pyrolysis Using PbSe Colloidal Solutions.

This work describes the spray pyrolysis deposition of PbSe films, using as-prepared PbSe colloids as the starting solution. The PbSe colloids were prepared by using the alkahest approach, where Pb and Se precursors were made to react with the following green polyols: glycerin, ethylene glycol, and propylene glycol, to subsequently spray them onto glass substrates. The results of the characterization indicated that amine or thiol groups-free and single-phase rock-salt cubic PbSe powder was obtained, producing nanocrystals 16-30 nm in size. X-ray diffraction also showed that the PbSe films containing PbSeO3 and PbO·xH2O as impurity phases were produced during the deposition. The morphology of the powders and films was developed by a self-assembly process, in which the primary PbSe nanoparticles self-assemble to produce peanut-like microstructures. Additionally, a non-continuous and porous feature was formed in the thick films. Certain films revealed optical structures characterized by broad- and low-intensity bands resembling an exciton-like behavior. This could be attributed to the presence of nanocrystals with a size less than the Bohr radius, indicating reminiscent quantum effects. The results suggest that the usage of colloidal dispersions as spray solutions represents an effective approach to forming PbSe films, as well as that the synthesis method allows for the elimination of thiol and amine groups before deposition, significantly simplifying the process.

PbSe/PbS Core/Shell Nanoplatelets with Enhanced Stability and Photoelectric Properties.

Lead chalcogenide nanoplatelets (NPLs) have emerged as a promising material for devices operating in the near IR and IR spectrum region. Here, we first apply the cation exchange method to PbSe/PbS core/shell NPL synthesis. The shell growth enhances NPL colloidal and environmental stability, and passivates surface trap states, preserving the main core physical properties. To prove the great potential for optoelectrical applications, we fabricate a photoconductor using PbSe/PbS NPLs. The device demonstrates enhanced conductivity and responsivity with fast rise and fall times, resulting in a 13 kHz bandwidth. The carrier transport was investigated with the field effect transistor method, showing p-type conductivity with charge mobility of 1.26 × 10-2 cm2·V-1·s-1.

High-Performance Uncooled Mid-Infrared Detector Based on a Polycrystalline PbSe/CdSe Heterojunction.

Developing high-performance, uncooled mid-wavelength infrared (MWIR) detectors is a challenging task due to the inherent physical properties of materials and manufacturing technologies. In this study, we designed and manufactured an uncooled polycrystalline PbSe/CdSe heterojunction photovoltaic (PV) detector through vapor physical deposition. The resulting 10 µm × 10 µm device exhibited a peak detectivity of 7.5 × 109 and 3 × 1010 cm·Hz1/2·W-1 at 298 and 220 K, respectively, under blackbody radiation. These values are comparable to those of typical PbSe photoconductive detectors fabricated through standard chemical bath deposition. Additionally, the sensitization-free process used to create these PbSe/CdSe PV detectors allows for high replicability and yield, making them promising candidates for low-cost, high-performance, uncooled MWIR focal plane array imaging in commercial applications.

Large-Scale Colloidal Synthesis of Chalcogenides for Thermoelectric Applications.

A simple and effective preparation of solution-processed chalcogenide thermoelectric materials is described. First, PbTe, PbSe, and SnSe were prepared by gram-scale colloidal synthesis relying on the reaction between metal acetates and diphenyl dichalcogenides in hexadecylamine solvent. The resultant phase-pure chalcogenides consist of highly crystalline and defect-free particles with distinct cubic-, tetrapod-, and rod-like morphologies. The powdered PbTe, PbSe, and SnSe products were subjected to densification by spark plasma sintering (SPS), affording dense pellets of the respective chalcogenides. Scanning electron microscopy shows that the SPS-derived pellets exhibit fine nano-/micro-structures dictated by the original morphology of the key constituting particles, while the powder X-ray diffraction and electron microscopy analyses confirm that the SPS-derived pellets are phase-pure materials, preserving the structure of the colloidal synthesis products. The resultant solution-processed PbTe, PbSe, and SnSe exhibit low thermal conductivity, which might be due to the enhanced phonon scattering developed over fine microstructures. For undoped n-type PbTe and p-type SnSe samples, an expected moderate thermoelectric performance is achieved. In contrast, an outstanding figure-of-merit of 0.73 at 673 K was achieved for undoped n-type PbSe outperforming, the majority of the optimized PbSe-based thermoelectric materials. Overall, our findings facilitate the design of efficient solution-processed chalcogenide thermoelectrics.

Simultaneous removal of lead and selenium through biomineralization as lead selenide by anaerobic granular sludge.

This study demonstrated the simultaneous removal of lead (Pb) and selenium (Se) as lead selenide biomineralization using anaerobic granular sludge. The microbial community of the granular sludge was first enriched for 140 days in the presence of Pb(II) only, selenate and selenite only, Pb(II)+selenate, and Pb(II)+selenite. In the absence of Se, removal of Pb(II) mainly occurred via biosorption and deposited on the biomass as lead oxide and lead carbonate. The Pb removal efficiency (94% of initial 50 mg L-1) was reduced to 90% and 86% in the presence of selenate and selenite, respectively, due to biosorption. Addition of Pb(II) didn't exert any toxic effect on the Se-reducing microbial community, on the contrary: Pb(II) addition improved the Se removal efficiency for selenate from 85% to 90%, but did not affect selenite removal after 14 d of incubation. The bioreduction of the Se-oxyanions produced elemental Se (Se(0)) and selenide, which later interacted with Pb(II) to produce lead selenide (PbSe). Adsorption of Pb(II) onto the Se(0) nanoparticles and precipitation as the Se(0)-Pb complex might also have contributed to the simultaneous removal of Pb and Se. XPS and XRD analysis further confirmed the immobilization of Pb as PbSe, PbO and PbCO3 in the biomass.

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.

Temperature-Dependent Photoluminescent Properties of PbSe Nanoplatelets.

Semiconductor colloidal nanoplatelets (NPLs) are a promising new class of nanostructures that can bring much impact on lightning technologies, light-emitting diodes (LED), and laser fabrication. Indeed, great progress has been made in optimizing the optical properties of the NPLs for the visible spectral range, which has already made the implementation of a number of effective devices on their basis possible. To date, state-of-the-art near-infrared (NIR)-emitting NPLs are significantly inferior to their visible-range counterparts, although it would be fair to say that they received significantly less research attention so far. In this study, we report a comprehensive analysis of steady-state and time-dependent photoluminescence (PL) properties of four monolayered (ML) PbSe NPLs. The PL measurements are performed in a temperature range of 78-300 K, and their results are compared to those obtained for CdSe NPLs and PbSe quantum dots (QDs). We show that multiple emissive states, both band-edge and trap-related, are responsible for the formation of the NPLs' PL band. We demonstrate that the widening of the PL band is caused by the inhomogeneous broadening rather than homogeneous one, and analyze the possible contributions to PL broadening.

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.

Charge Transport Modulation in PbSe Nanocrystal Solids by Au xAg1- x Nanoparticle Doping.

Nanocrystal (NC) solids are an exciting class of materials, whose physical properties are tunable by choice of the NCs as well as the strength of the interparticle coupling. One can consider these NCs as "artificial atoms" in analogy to the formation of condensed matter from atoms. Akin to atomic doping, the doping of a semiconducting NC solid with impurity NCs can drastically alter its electronic properties. A high degree of complexity is possible in these artificial structures by adjusting the size, shape, and composition of the building blocks, which enables "designer" materials with targeted properties. Here, we present the doping of the PbSe NC solids with a series of Au xAg1- x alloy nanoparticles (NPs). A combination of temperature-dependent electrical conductance and Seebeck coefficient measurements and room-temperature Hall effect measurements demonstrates that the incorporation of metal NPs both modifies the charge carrier density of the NC solids and introduces energy barriers for charge transport. These studies point to charge carrier injection from the metal NPs into the PbSe NC matrix. The charge carrier density and charge transport dynamics in the doped NC solids are adjustable in a wide range by employing the Au xAg1- x NP with different Au:Ag ratio as dopants. This doping strategy could be of great interest for thermoelectric applications taking advantage of the energy filtering effect introduced by the metal NPs.

Interface and Strain Energy Revolution Texture Map To Predict Structure and Optical Properties of Sputtered PbSe Thin Films.

The preferred growth orientation of the sputtered lead selenide (PbSe) thin films on Si(100) substrates was thermodynamically simulated and calculated on the basis of the density functional theory. The results showed that the total free energy variation during the grain growth is dominated by the interface and strain energy minimization under certain conditions, indicating that the preferred growth orientation and related optical properties of the PbSe thin films can be effectively modified by these two energy variations. Thermodynamically, the PbSe[200] and PbSe[220] preferred orientations are obtained when the interface and strain energy minimization dominate the total free energy variation, respectively. A texture map related to the interface and strain energy revolution was obtained, which can be used to predict the structure and optical properties of the sputtered PbSe thin films, and its applicability was confirmed by the real X-ray diffraction and Fourier transform infrared spectroscopy experimental results of four midfrequency sputtered PbSe thin films with designed thickness and microstrain deposited on Si(100) substrates.

Lead selenide-Titanium dioxide heteronanojunction formation by photocatalytic current doubling-induced two-step photodeposition technique.

PbSe quantum dots (QDs) were formed on TiO2 by a two-step photodeposition technique. At the first step, UV-light irradiation of TiO2 in an ethanol solution of H2SeO3 yields Se QDs on the TiO2 surface in a highly dispersed state (Se/TiO2). At the second step, UV-light irradiation of Se/TiO2 in an ethanol solution of Pb(ClO4)2 transforms Se QDs into several tens of nanometer-sized cubic deposits identified as PbSe (PbSe/TiO2) by X-ray diffraction, electronic absorption measurements and X-ray photoelectron spectroscopy. Photochronopotentiometry measurements suggested that the PbSe QDs are formed on TiO2 via the Pb(2+) ion-assisted reduction of Se particles.

Lifetime, mobility, and diffusion of photoexcited carriers in ligand-exchanged lead selenide nanocrystal films measured by time-resolved terahertz spectroscopy.

Colloidal semiconductor nanocrystals have been used as building blocks for electronic and optoelectronic devices ranging from field-effect transistors to solar cells. Properties of the nanocrystal films depend sensitively on the choice of capping ligand to replace the insulating synthesis ligands. Thus far, ligands leading to the best performance in transistors result in poor solar cell performance, and vice versa. To gain insight into the nature of this dichotomy, we used time-resolved terahertz spectroscopy measurements to study the mobility and lifetime of PbSe nanocrystal films prepared with five common ligand-exchange reagents. Noncontact terahertz spectroscopy measurements of conductivity were corroborated by contacted van der Pauw measurements of the same samples. The films treated with different displacing ligands show more than an order of magnitude difference in the peak conductivities and a bifurcation of time dynamics. Inorganic chalcogenide ligand exchanges with sodium sulfide (Na2S) or ammonium thiocyanate (NH4SCN) show high mobilities but nearly complete decay of transient photocurrent in 1.4 ns. In contrast, ligand exchanges with 1,2-ethylenediamine (EDA), 1,2-ethanedithiol (EDT), and tetrabutylammonium iodide (TBAI) show lower mobilities but longer carrier lifetimes, resulting in longer diffusion lengths. This bifurcated behavior may explain the divergent performance of field-effect transistors and photovoltaics constructed from nanocrystal building blocks with different ligand exchanges.

p-i-n Heterojunction solar cells with a colloidal quantum-dot absorber layer.

A quantum-dot (QD) p-i-n heterojunction solar cell with an increased depletion region is demonstrated by depleting the QD layer from both the front and back junctions. Due to a combination of improved charged extraction and increased light absorption, a 120% increase in the short-circuit current is achieved compared with that of conventional ZnO/QD devices.

Effect of deposition temperature on the structural and optical properties of chemically prepared nanocrystalline lead selenide thin films.

Nanocrystalline lead selenide (PbSe) thin films were prepared on glass substrates by a chemical bath deposition method, using sodium selenosulfate (Na(2)SeSO(3)) as a source of Se(2-) ions, and lead acetate as a source of Pb(2+) ions. Trisodium citrate (TSC) was used as a complexing agent. PbSe films were prepared at various deposition temperatures while the pH value was kept fixed at 11, and the effect on the resulting film properties was studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and optical absorption studies. The structural parameters, such as the lattice constant (a), crystallite size (D), dislocation density (ρ) and microstrain (ε) were evaluated from the XRD spectra. It was found that average crystallite size, as calculated from Scherrer's formula, increased from 23 to 33 nm as the deposition temperature was varied from 303 to 343 K. The dislocation density and microstrain were found to vary inversely with the crystallite size, whereas the lattice constant was found to increase with an increase in crystallite size. The optical absorption spectra of the nanocrystalline PbSe films showed a blue shift, and the optical band gap (E(g)) was found to increase from 1.96 to 2.10 eV with the decrease in crystallite size.