Reversible Surface Electronic Traps in PbS Quantum Dot Solids Induced by an Order–Disorder Phase Transition in Capping Molecules

The electronic properties of semiconductor quantum dots (QDs) are critically dependent on the nature of the ligand molecules on their surfaces. Here we show the reversible formation of surface electronic trap states in the model system of solid thin films of PbS QDs capped with thiol molecules. As the temperature was increased from cryogenic to room temperature, we discovered a phase transition in the fluorescence spectra from excitonic emission to trap emission. The critical temperature (<i>T</i>_c) of the phase transition scales with molecular length and in each case is close to the bulk melting temperature of the capping molecules. We conclude that an order–disorder transition in the molecular monolayer above <i>T</i>_c introduces surface mobility and the formation of a disordered atomic lead layer at the QD/capping molecule interface, leading to electronic trap formation

Template-Assembly of Quantum Dot Molecules

Semiconductor quantum dots (QDs) have been called artificial atoms because of their discrete electronic structures. Assembling them into artificial molecules may greatly expand our capability in controlling physical properties on the nanoscale. Here we show the successful assembly and size control of colloidal PbSe QD clusters into large-scale templates defined by block-copolymer patterns. Following the exchange of capping molecules, the QD clusters behave as artificial molecules due to enhanced and local electronic interactions

Excitonic Many-Body Interactions in Two-Dimensional Lead Iodide Perovskite Quantum Wells

While the perovskite fever has focused on three-dimensional crystalline solids, this class of material can also self-assemble into two-dimensional (2D) layered structures that are natural quantum wells with tunable thickness and optoelectronic properties. Here we apply femtosecond transient absorption spectroscopy to study the many-body optical responses of 2D perovskites with the general formula of (C₄H₉NH₃I)₂(CH₃NH₃I)_<i>n</i>−1(PbI₂)_<i>n</i>, where <i>n</i> = 1, 2, 3) is the number of lead iodide unit cells in the direction perpendicular to the 2D quantum well. In the thinnest quantum well (<i>n</i> = 1), above-gap optical excitation induces a blue shift but no population bleaching at the excitonic resonance; this is similar to the many-body optical response of conventional inorganic quantum wells. In contrast to inorganic quantum wells, we find the excitonic blue-shift in 2D perovskites to be independent of excitation power density. We take this as evidence for a Mott-Wannier exciton localizing into a “puddle”, which only exerts local influence on subsequent optical excitations. The excitonic puddles likely come from the disordered electronic energy landscape expected for the soft 2D hybrid organic–inorganic perovskite lattice. As the thickness of the quantum well increases to <i>n</i> = 3, free carrier characters start to show up for above band gap excitation; this is reflected in the broadening and bleaching of the excitonic resonance (in addition to blue-shift), attributed to carrier-exciton collision and screening of the Coulomb potential, respectively

Rational Fabrication of Arrays of Plasmonic Metal–Quantum Dot Sandwiched Nanodisks with Enhanced Förster Resonance Energy Transfer

The size-dependent optical gap due to the quantum confinement effect is a hallmark of semiconductor quantum dots (QDs). A major research effort underway is to control energy transfer in QDs for diverse applications ranging from solar energy harvesting to fluorescent displays and biological imaging. In this work, we report an innovative approach for fabricating monodispersed arrays of Au/polymer/QDs sandwiched nanodisks on a wafer scale with high rationality and reproducibility. Via template-assisted assembling of QDs, the plasmonic Au nanodisk can be precisely coupled to a defined number of QD clusters. Using steady-state and time-resolved photoluminescence spectroscopy, we studied the enhancement of Förster resonance energy transfer (FRET) rate between donor and acceptor QDs coupled with plasmonic resonance of the Au nanodisks. By systematically increasing the spectral overlap of the acceptor QD emission with plasmonic resonance, we demonstrated that the donor lifetime decreases monotonically and FRET rate increases significantly. The results suggest a viable route to fabricate precisely controlled plasmonic–QD coupled nanoentities and to enhance energy transfer at the nanoscale

Harvesting Singlet Fission for Solar Energy Conversion: One- versus Two-Electron Transfer from the Quantum Mechanical Superposition

Singlet fission, the creation of two triplet excitons from one singlet exciton, is being explored to increase the efficiency of solar cells and photo detectors based on organic semiconductors, such as pentacene and tetracene. A key question is how to extract multiple electron–hole pairs from multiple excitons. Recent experiments in our laboratory on the pentacene/C₆₀ system (Chan, W.-L.; et al. <i>Science</i> <b>2011</b>, <i>334</i>, 1543–1547) provided preliminary evidence for the extraction of two electrons from the multiexciton (ME) state resulting from singlet fission. The efficiency of multielectron transfer is expected to depend critically on other dynamic processes available to the singlet (S₁) and the ME, but little is known about these competing channels. Here we apply time-resolved photoemission spectroscopy to the tetracene/C₆₀ interface to probe one- and two-electron transfer from S₁ and ME states, respectively. Unlike ultrafast (∼100 fs) singlet fission in pentacene where two-electron transfer from the multiexciton state resulting from singlet fission dominates, the relatively slow (∼7 ps) singlet fission in tetracene allows both one- and two-electron transfer from the S₁ and the ME states that are in a quantum mechanical superposition. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photoexcited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C₆₀, respectively. Kinetic analysis shows that ∼60% of the S₁ ⇔ ME quantum superposition transfers one electron through the S₁ state to C₆₀ while ∼40% undergoes two-electron transfer through the ME state. We discuss design principles at donor/acceptor interfaces for optimal multiple carrier extraction from singlet fission for solar energy conversion

A Mobile Precursor Determines Amyloid‑β Peptide Fibril Formation at Interfaces

The aggregation of peptides into amyloid fibrils plays a crucial role in various neurodegenerative diseases. While it has been generally recognized that fibril formation in vivo may be greatly assisted or accelerated by molecular surfaces, such as cell membranes, little is known about the mechanism of surface-mediated fibrillation. Here we study the role of adsorbed Alzheimer’s amyloid-β peptide (Aβ42) on surface-mediated fibrillation using polymer coatings of varying hydrophobicity as well a supported lipid bilayer membrane. Using single molecule fluorescent tracking and atomic force microscopy imaging, we show that weakly adsorbed peptides with two-dimensional diffusivity are critical precursors to fibril growth on surfaces. This growth mechanism is inhibited on the highly hydrophilic surface where the surface coverage of adsorbed peptides is negligible or on the highly hydrophobic surface where the diffusion constant of the majority of adsorbed peptides is too low. Physical properties that favor weakly adsorbed peptides with sufficient translational mobility can locally concentrate peptide molecules on the surface and promote inter-peptide interaction via two-dimensional confinement, leading to fibrillation at Aβ peptide concentration many orders of magnitude below the critical concentration for fibrillation in the bulk solution

A Mobile Precursor Determines Amyloid‑β Peptide Fibril Formation at Interfaces

The aggregation of peptides into amyloid fibrils plays a crucial role in various neurodegenerative diseases. While it has been generally recognized that fibril formation in vivo may be greatly assisted or accelerated by molecular surfaces, such as cell membranes, little is known about the mechanism of surface-mediated fibrillation. Here we study the role of adsorbed Alzheimer’s amyloid-β peptide (Aβ42) on surface-mediated fibrillation using polymer coatings of varying hydrophobicity as well a supported lipid bilayer membrane. Using single molecule fluorescent tracking and atomic force microscopy imaging, we show that weakly adsorbed peptides with two-dimensional diffusivity are critical precursors to fibril growth on surfaces. This growth mechanism is inhibited on the highly hydrophilic surface where the surface coverage of adsorbed peptides is negligible or on the highly hydrophobic surface where the diffusion constant of the majority of adsorbed peptides is too low. Physical properties that favor weakly adsorbed peptides with sufficient translational mobility can locally concentrate peptide molecules on the surface and promote inter-peptide interaction via two-dimensional confinement, leading to fibrillation at Aβ peptide concentration many orders of magnitude below the critical concentration for fibrillation in the bulk solution

A Mobile Precursor Determines Amyloid‑β Peptide Fibril Formation at Interfaces

The aggregation of peptides into amyloid fibrils plays a crucial role in various neurodegenerative diseases. While it has been generally recognized that fibril formation in vivo may be greatly assisted or accelerated by molecular surfaces, such as cell membranes, little is known about the mechanism of surface-mediated fibrillation. Here we study the role of adsorbed Alzheimer’s amyloid-β peptide (Aβ42) on surface-mediated fibrillation using polymer coatings of varying hydrophobicity as well a supported lipid bilayer membrane. Using single molecule fluorescent tracking and atomic force microscopy imaging, we show that weakly adsorbed peptides with two-dimensional diffusivity are critical precursors to fibril growth on surfaces. This growth mechanism is inhibited on the highly hydrophilic surface where the surface coverage of adsorbed peptides is negligible or on the highly hydrophobic surface where the diffusion constant of the majority of adsorbed peptides is too low. Physical properties that favor weakly adsorbed peptides with sufficient translational mobility can locally concentrate peptide molecules on the surface and promote inter-peptide interaction via two-dimensional confinement, leading to fibrillation at Aβ peptide concentration many orders of magnitude below the critical concentration for fibrillation in the bulk solution

A Mobile Precursor Determines Amyloid‑β Peptide Fibril Formation at Interfaces

The aggregation of peptides into amyloid fibrils plays a crucial role in various neurodegenerative diseases. While it has been generally recognized that fibril formation in vivo may be greatly assisted or accelerated by molecular surfaces, such as cell membranes, little is known about the mechanism of surface-mediated fibrillation. Here we study the role of adsorbed Alzheimer’s amyloid-β peptide (Aβ42) on surface-mediated fibrillation using polymer coatings of varying hydrophobicity as well a supported lipid bilayer membrane. Using single molecule fluorescent tracking and atomic force microscopy imaging, we show that weakly adsorbed peptides with two-dimensional diffusivity are critical precursors to fibril growth on surfaces. This growth mechanism is inhibited on the highly hydrophilic surface where the surface coverage of adsorbed peptides is negligible or on the highly hydrophobic surface where the diffusion constant of the majority of adsorbed peptides is too low. Physical properties that favor weakly adsorbed peptides with sufficient translational mobility can locally concentrate peptide molecules on the surface and promote inter-peptide interaction via two-dimensional confinement, leading to fibrillation at Aβ peptide concentration many orders of magnitude below the critical concentration for fibrillation in the bulk solution

A Mobile Precursor Determines Amyloid‑β Peptide Fibril Formation at Interfaces

The aggregation of peptides into amyloid fibrils plays a crucial role in various neurodegenerative diseases. While it has been generally recognized that fibril formation in vivo may be greatly assisted or accelerated by molecular surfaces, such as cell membranes, little is known about the mechanism of surface-mediated fibrillation. Here we study the role of adsorbed Alzheimer’s amyloid-β peptide (Aβ42) on surface-mediated fibrillation using polymer coatings of varying hydrophobicity as well a supported lipid bilayer membrane. Using single molecule fluorescent tracking and atomic force microscopy imaging, we show that weakly adsorbed peptides with two-dimensional diffusivity are critical precursors to fibril growth on surfaces. This growth mechanism is inhibited on the highly hydrophilic surface where the surface coverage of adsorbed peptides is negligible or on the highly hydrophobic surface where the diffusion constant of the majority of adsorbed peptides is too low. Physical properties that favor weakly adsorbed peptides with sufficient translational mobility can locally concentrate peptide molecules on the surface and promote inter-peptide interaction via two-dimensional confinement, leading to fibrillation at Aβ peptide concentration many orders of magnitude below the critical concentration for fibrillation in the bulk solution