PbSe Nanocrystal Quantum Dots Research Articles

Ultrastable PbSe nanocrystal quantum dots via in situ formation of atomically thin halide adlayers on PbSe(100).

The fast degradation of lead selenide (PbSe) nanocrystal quantum dots (NQDs) in ambient conditions impedes widespread deployment of the highly excitonic, thus versatile, colloidal NQDs. Here we report a simple in situ post-synthetic halide salt treatment that results in size-independent air stability of PbSe NQDs without significantly altering their optoelectronic characteristics. From TEM, NMR, and XPS results and DFT calculations, we propose that the unprecedented size-independent air stability of the PbSe NQDs can be attributed to the successful passivation of under-coordinated PbSe(100) facets with atomically thin PbX2 (X = Cl, Br, I) adlayers. Conductive films made of halide-treated ultrastable PbSe NQDs exhibit markedly improved air stability and behave as an n-type channel in a field-effect transistor. Our simple in situ wet-chemical passivation scheme will enable broader utilization of PbSe NQDs in ambient conditions in many optoelectronic applications.

Highly Effective Surface Passivation of PbSe Quantum Dots through Reaction with Molecular Chlorine

PbSe nanocrystal quantum dots (NQDs) are a promising active material for a range of optoelectronic devices, including solar cells, high-sensitivity infrared (IR) photodetectors, and IR-emitting diodes and lasers. However, device realization has been constrained by these NQDs' chemical instability toward oxidation, which leads to uncontrollable changes in optical and electronic properties. Here, we present a simple method to enhance the stability of PbSe NQDs against oxidation and to improve their optical properties through reaction with molecular chlorine. The chlorine molecules preferentially etch out surface Se ions and react with Pb ions to form a thin (1-2 monolayers) PbCl(x) passivation layer which effectively prevents oxidation during long-term air exposure while passivating surface trap states to increase photoluminescence efficiency and decrease photocharging. Our method is simple, widely applicable to PbSe and PbS NQDs of a range of sizes, compatible with solution-based processes for fabricating NQD-based devices, and effective both in solution and in solid NQD films; thus, it is a practical protocol for facilitating advances over the full range of optoelectronic applications.

Spectral Dependence of Nanocrystal Photoionization Probability: The Role of Hot-Carrier Transfer

We conduct measurements of photocharging of PbSe and PbS nanocrystal quantum dots (NQDs) as a function of excitation energy (ℏω). We observe a rapid growth of the degree of photocharging with increasing ℏω, which indicates an important role of hot-carrier transfer in the photoionization process. The corresponding spectral dependence exhibits two thresholds that mark the onsets of weak and strong photocharging. Interestingly, both thresholds are linked to the NQD band gap energy (E(g)) and scale as ∼1.5E(g) and ∼3E(g), indicating that the onsets of photoionization are associated with specific nanocrystal states (tentatively, 1P and 2P, respectively) and are not significantly dependent on the energy of external acceptor sites. For all samples, the hot-electron transfer probability increases by nearly 2 orders of magnitude as photon energy increases from 1.5 to 3.5 eV, although at any given wavelength the photoionization probability shows significant sample-to-sample variations (∼10(-6) to 10(-3) for 1.5 eV and ∼10(-4) to 10(-1) for 3.5 eV). In addition to the effect of the NQD size, these variations are likely due to differences in the properties of the NQD surface and/or the number and identity of external acceptor trap sites. The charge-separated states produced by photoionization are characterized by extremely long lifetimes (20 to 85 s) that become longer with increasing NQD size.

Efficient Hybrid Infrared Solar Cells Based on P3HT and PbSe Nanocrystal Quantum Dots

The effects of composition on the performance of hybrid photovoltaic cells based on PbSe nanocrystal quantum dot(NQD)/ poly(3-hexylthiophene) nanocomposites were studied. The device performance and the absorption property strongly depended on the loading of PbSe. To improve the device performance, a pure light-absorbing P3HT film was inserted underneath the blended layer of P3HT and PbSe nanocrystal quantum dots in the active region. Such a design allowed for the employment of a thinner bulk heterojunction for more efficient carrier collection without an excessive reduction of the overall light absorption by the photovoltaic cell. The measured device performance showed a significant improvement over previously reported hybrid cells containing bulk heterojunctions of P3HT and Pb(S,Se) nanocrystal quantum dots.

Electroabsorption spectra of PbSe nanocrystal quantum dots

The quantum-confined Stark effect of PbSe nanocrystal quantum dots (QDs) in a polymer film has been studied for the near-infrared absorption in the region of 1300–1600 nm (corresponding to diameters of 4.0–7.0 nm) by using electric field modulation spectroscopy. Electroabsorption spectra of QDs show the electric-field-induced energy shift in the exciton state, resulting in the spectral broadening given by the second derivative of the absorption spectra. It is shown that the magnitude of the change in electric dipole moment following excitation into the first exciton state increases with an increase in the size of QDs.

Integration of planar and bulk heterojunctions in polymer/nanocrystal hybrid photovoltaic cells

We present in this communication a hybrid polymer/nanocrystal photovoltaic device architecture wherein a net poly(3-hexylthiophene) (P3HT) light-absorbing film is inserted underneath the blended layer of P3HT and PbSe nanocrystal quantum dots in the active region. Such a design features the vertical integration of planar and bulk heterojunctions, which allows for the employment of a thinner bulk heterojunction for more efficient carrier collection without an excessive reduction of the overall light absorption by the photovoltaic cell. The measured device performance represents a significant improvement over previously reported hybrid cells containing bulk heterojunctions of P3HT and Pb(S,Se) nanocrystal quantum dots.

A Reduction Pathway in the Synthesis of PbSe Nanocrystal Quantum Dots

Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general interest because of their unparalleled potential as infrared fluorophores. While PbSe NQDs are a promising class of infrared-active nanocrystals due to high emission quantum yields and a wide useful spectral range, typical synthetic methods are sensitive to a variety of factors, including the influence of solvent/ligand impurities that render reproducibility difficult. In this work, we specifically examine the effects of diphenylphosphine and 1,2-hexadecanediol, as surrogates for putative trioctylphosphine-based reducing impurities, on the synthesis of PbSe NQDs. Specifically, we compare their influence on NQD size, chemical yield, and photoluminescence quantum yield. While both additives substantially increase the chemical yield of the synthesis, they demonstrate markedly different effects on emission quantum yield of the product NQDs. We further examine the effects of reaction temperature and oleic acid concentration on the diol-assisted synthesis. Increased oleic acid concentration led to somewhat higher growth rates and larger NQDs but at the expense of lower chemical yield. Temperature was found to have an even greater effect on growth rate and NQD size. Neither temperature nor oleic acid concentration was found to have noticeable effects on NQD emission quantum yield. Finally, we use numerical simulations to support the conjecture that the increased yield is likely a result of faster monomer formation, consistent with the activation of an additional reaction pathway by the reducing species.

Selection rules for optical transitions in PbSe nanocrystal quantum dots: Drastic effect of structure inversion asymmetry

We resolve a paradox according to which optical transitions seemingly forbidden in PbSe nanocrystal quantum dots by the parity selection rule exhibit large oscillator strengths in optical absorption. The transitions become allowed if the center of a spherical nanocrystal is shifted away from an atomic site leading to the structure inversion asymmetry. This asymmetry is accounted for within a four-band envelope-function formalism. Transition strengths for various optical transitions are calculated.

Selection Rules for Optical Transitions in PbSe Nanocrystal Quantum Dots: Drastic Effect of Structure Inversion Asymmetry

We resolve a paradox according to which optical transitions seemingly forbidden in PbSe nanocrystal quantum dots by the parity selection rule exhibit large oscillator strengths in optical absorption. The transitions become allowed if the center of a spherical nanocrystal is shifted away from an atomic site leading to the structure inversion asymmetry. This asymmetry is accounted for within a four-band envelope-function formalism. Transition strengths for various optical transitions are calculated.

Selection Rules for Optical Transitions in PbSe Nanocrystal Quantum Dots: Drastic Effect of Structure Inversion Asymmetry

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The ground-state exciton lifetime of PbSe nanocrystal quantum dots

We have investigated the ground-state exciton lifetime of PbSe nanocrystal quantum dots (NQDs). We found, under variation NQDs sizes, surrounding temperature and media, that the ground-state exciton lifetime of PbSe core NQDs showed a steep decrease of the lifetime from 6 to 1 μs when the temperature is changed from 7 to 150 K, while above that temperature, there is only a minor change in the measured lifetime, which are influenced of the surrounding medium.

Utilizing the Lability of Lead Selenide to Produce Heterostructured Nanocrystals with Bright, Stable Infrared Emission

Infrared-emitting nanocrystal quantum dots (NQDs) have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Notably, lead chalcogenide NQDs, especially PbSe NQDs, provide efficient emission over a large spectral range in the infrared, but their application has been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. Conventional methods for improving NQD stability by applying a shell of a more stable, wider band gap semiconductor material are frustrated by the tendency of lead chalcogenide NQDs toward Ostwald ripening at even moderate reaction temperatures. Here, we describe a partial cation-exchange method in which we take advantage of this lability to controllably synthesize PbSe/CdSe core/shell NQDs. Critically, these NQDs are stable against fading and spectral shifting. Further, these NQDs can undergo additional shell growth to produce PbSe/CdSe/ZnS core/shell/shell NQDs that represent initial steps toward bright, biocompatible near-infrared optical labels.

Coherent defect-assisted multiphonon intraband carrier relaxation in semiconductor quantum dots

A defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots.

Multiple exciton generation and electrical extraction from a PbSe quantum dot photoconductor

Multiple exciton generation and subsequent electrical extraction from a thin film photoconductive device constructed from PbSe nanocrystal quantum dots are demonstrated. The hydrazine treatment of the PbSe film drastically improves the conductivity of the film while maintaining excellent optical and structural film quality. The effects of multiple exciton generation and electrical extraction (electrons collected per photon absorbed) were quantified as a function of incident photon energy from 1.55to3.1eV. The multiple carrier extraction (>100%) was observed at photon energies greater than 2.8 times of the quantum dot bandgap with ∼210% measured at 4.4 times the bandgap.

Binding Characteristics of Surface Ligands on PbSe QDs and Impact on Electrical Conductivity

ABSTRACTWe report the binding and conductivity characteristics of PbSe nanocrystal quantum dots which have different ligands on the surface. The PbSe nanocrystal quantum dots were surface functionalized using chemical treatment. The results of post-treatment analysis show that the PbSe surface can be successfully functionalized with various ligands based on thiol- and amine-molecules (e.g., oleic acid, dodecanethiol and butylamine). Conductivity measurements performed using a metal-semiconductor-metal structure indicate that there is an increase of conductivity as the length of the ligand connecting the quantum dots is reduced.

Coherent multiphonon energy relaxation in a quantum dot

A new defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots.

Hierarchical Conjugate Structure of γ-Fe2O3 Nanoparticles and PbSe Quantum Dots for Biological Applications

The development and characterization of a unique conjugated structure comprised of γ-Fe2O3 nanoparticles (NPs) and PbSe nanocrystal quantum dots (NQDs), bonded via chemical functional groups, are discussed. Compatibility with an aqueous environment was rendered by exchanging the traditional organic surfactants (oleic acid) of the PbSe NQDs and γ-Fe2O3 NPs with 2-aminoethanethiol and polyhedral silsesquioxane hydrate octakis ligands, respectively. Comparison of the transmission electron microscopy (TEM) images of the PbSe NQDs and γ-Fe2O3 NPs capped with either organic or water-soluble ligands highlights the preservation of the morphology and size of both types of nanoclusters upon ligand exchange. Furthermore, the absorbance, photoluminescence (PL), and Fourier transform infrared spectra of the PbSe QDs revealed that the optical properties of the NQDs were retained during the surfactant exchange. High-resolution TEM images of the complete conjugated structure validate the presence of the coupling between ...

Developing PbSe/PbS core-shell nanocrystals quantum dots toward their potential heterojunction applications

We report in this article the epitaxial growth of PbS shells over PbSe nanocrystal quantum dots (NQDs) with monolayer-precision. The core-shell infrared NQDs were ligand-exchanged with short-chain octylamine, and the photoluminescence efficiency of the surface-engineered core-shell nanoparticles was substantially higher than that of the plain-core structures undergoing the same surface processing, which reveals less ligand-dependence and enhanced chemical robustness in the core-shell NQDs. The reported results could potentially open up the possibility of incorporating semiconductor infrared NQDs in the silicon matrix to develop all-inorganic light emitting heterojunctions on silicon substrates.

Optical properties of PbSe nanocrystal quantum dots under pressure

The optical properties of PbSe nanocrystal quantum dots (NQDs) were studied as a function of applied hydrostatic pressure over the range from ambient to 5.4GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement. Despite such strong confinement, the authors find that the energy gaps of 3, 5, and 7nm diameter PbSe NQDs change monotonically with pressure with a dependence that is almost entirely determined by the bulk deformation potential. The sizable dependence of the NQD energy gap with pressure invites applications in the areas of high speed pressure sensing and tunable IR lasers.

Synthesis and surface modification of PbSe/PbS core–shell nanocrystals for potential deviceapplications

We report in this paper the growth of PbS shells over colloidal PbSe nanocrystal quantumdots (NQDs) with monolayer-precision. The technique of successive ion layeradsorption and reaction (SILAR) has been adapted to the growth of high-qualitycore–shell nanocrystals. The core–shell infrared NQDs were ligand-exchanged withshort-chain octylamine, and the photoluminescence efficiency of the surface-engineeredcore–shell nanoparticles was substantially higher than that of the plain core structuresundergoing the same surface processing, which reveals less ligand dependence andenhanced chemical robustness in the core–shell NQDs. The reported results openup the possibility of incorporating semiconductor infrared NQDs in the siliconmatrix to develop all-inorganic light-emitting heterojunctions on silicon substrates.