Versatile PbS Quantum Dot Ligand Exchange Systems in the Presence of Pb-Thiolates

Thickness-variation-insensitive near-infrared quantum dot LEDs

In terms of tunable luminescence and high quantum efficiency, colloidal quantum dots (CQDs) are promising semiconductors for constructing near-infrared light-emitting diodes (NIR-LEDs). However, currently available NIR-LEDs are susceptible to variations in the emission layer thickness (EMLT), the highest external quantum efficiency (EQE) decreases to below 50% (relative to peak EQE) when the EMLT varies out of a narrow range of (±30 nm). This is due to the thickness-dependent carrier recombination rate and current density variation, resulting in batch-to-batch EQE fluctuations that limit LED reproducibility. Here we report efficient NIR-LEDs that exhibit EQE variations of less than 15% (relative to the champion EQE) over an EMLT range of 40-220 nm; the highest achievable EQE of ∼11.5% was obtained by encapsulating a 212 nm-thick CQD within a type-I inorganic shell to enhance the radiative recombination in the dots, resulting in a high photoluminescence quantum yield of 80%, and by post-treating the films with a bifunctional linking agent to improve and balance the hole and electron mobilities in the entire film (electron mobility: 8.23 × 10-3 cm2 V-1 s-1; hole mobility: 7.0 × 10-3 cm2 V-1 s-1). This work presents the first NIR-LEDs that exhibit EMLT-invariant EQE over an EMLT range of 40-220 nm, which represents the highest EQE among reported CQD NIR-LEDs with a QD thickness exceeding 100 nm.

Lead Chalcogenide Colloidal Quantum Dots for Infrared Photodetectors

Infrared detection technology plays an important role in remote sensing, imaging, monitoring, and other fields. So far, most infrared photodetectors are based on InGaAs and HgCdTe materials, which are limited by high fabrication costs, complex production processes, and poor compatibility with silicon-based readout integrated circuits. This hinders the wider application of infrared detection technology. Therefore, reducing the cost of high-performance photodetectors is a research focus. Colloidal quantum dot photodetectors have the advantages of solution processing, low cost, and good compatibility with silicon-based substrates. In this paper, we summarize the recent development of infrared photodetectors based on mainstream lead chalcogenide colloidal quantum dots.

Characterizing the Semiconductor Nanocrystal Surface through Chemical Reactivity

ConspectusMany desirable and undesirable properties of semiconductor nanocrystals (NCs) can be traced to the NC surface due to the large surface-to-volume ratio. Therefore, precise control of the NC surface is imperative to achieve NCs with the desired qualities. Ligand-specific reactivity and surface heterogeneity make it difficult to accurately control and tune the NC surface. Without a molecular-level appreciation of the NC surface chemistry, modulating the NC surface is impossible and the risk of introducing deleterious surface defects is imminent. To gain a more comprehensive understanding of the surface reactivity, we have utilized a variety of spectroscopic techniques and analytical methods in concert.This Account describes our use of robust characterization techniques and ligand exchange reactions in effort to establish a molecular-level understanding of NC surface reactivity. The utility of NCs in target applications such as catalysis and charge transfer hangs on precise tunability of NC ligands. Modulating the NC surface requires the necessary tools to monitor chemical reactions. One commonly utilized analytical method to achieve targeted surface compositions is ¹H nuclear magnetic resonance (NMR) spectroscopy. Here we describe our use of ¹H NMR spectroscopy to monitor chemical reactions at CdSe and PbS NC surfaces to identify ligand specific reactivity. However, seemingly straightforward ligand exchange reactions can vary widely depending on the NC materials and anchoring group. Some non-native X-type ligands will irreversibly displace native ligands. Other ligands exist in equilibrium with native ligands. Depending on the application, it is important to understand the nature of exchange reactions. This level of understanding can be obtained by extracting exchange ratios, exchange equilibrium, and reaction mechanism information from ¹H NMR spectroscopy to establish precise NC reactivity.Reactivity that occurs through multiple, parallel ligand exchange mechanisms can involve both the liberation of metal-based Z-type ligands in addition to reactivity of X-type ligands. In these reactions, ¹H NMR spectroscopy fails to discern between an X-type oleate or a Z-type Pb(oleate)₂ because only the alkene resonance of the organic constituent is probed by this method. Multiple, parallel reaction pathways occur when thiol ligands are introduced to oleate-capped PbS NCs. This necessitated the use of synergistic characterization methods including ¹H NMR spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS) to characterize both surface-bound and liberated ligands.Similar analytical methods have been employed to probe the NC topology, which is an important, but often overlooked, component to NC reactivity given the facet-specific reactivity of PbS NCs. Through the tandem use of NMR spectroscopy and ICP-MS, we have monitored the liberation of Pb(oleate)₂ as an L-type ligand is titrated to the NC to determine the quantity and equilibrium of Z-type ligands. By studying a variety of NC sizes, we correlated the number of liberated ligands with the size-dependent topology of PbS NCs.Lastly, we incorporate redox-active chemical probes into our toolbox to study NC surface defects. We describe how the site-specific reactivity and relative energetics of redox-active surface-based defects are elucidated using redox probes and show that this reactivity is highly dependent on the surface composition. This Account is designed to encourage readers to consider the necessary characterization techniques needed establish a molecular-level understanding of NC surfaces in their own work.

Quantum Dot Transfer from the Organic Phase to Acrylic Monomers for the Controlled Integration of Single-Photon Sources by Photopolymerization

This paper reports on a new strategy for obtaining homogeneous dispersion of grafted quantum dots (QDs) in a photopolymer matrix and their use for the integration of single-photon sources by two-photon polymerization (TPP) with nanoscale precision. The method is based on phase transfer of QDs from organic solvents to an acrylic matrix. The detailed protocol is described, and the corresponding mechanism is investigated and revealed. The phase transfer is done by ligand exchange through the introduction of mono-2-(methacryloyloxy) ethyl succinate (MES) that replaces oleic acid (OA). Infrared (IR) measurements show the replacement of OA on the QD surface by MES after ligand exchange. This allows QDs to move from the hexane phase to the pentaerythritol triacrylate (PETA) phase. The QDs that are homogeneously dispersed in the photopolymer without any clusterization do not show any significant broadening in their photoluminescence spectra even after more than 3 years. The ability of the hybrid photopolymer to create micro- and nanostructures by two-photon polymerization is demonstrated. The homogeneity of emission from 2D and 3D microstructures is confirmed by confocal photoluminescence microscopy. The fabrication and integration of a single-photon source in a spatially controlled manner by TPP is achieved and confirmed by auto-correlation measurements.

Dielectric barrier discharge-accelerated one-pot synthesis of sulfur quantum dots for fluorescent sensing of lead ions and L-cysteine

Here, we report a novel method for the one-pot facile synthesis of sulfur quantum dots (SQDs) based on a dielectric barrier discharge (DBD)-accelerated H₂O₂ etching strategy within merely 20 min. The formation mechanism of SQDs was investigated, with which an "ON-OFF-ON" fluorescence sensor was developed for the detection of Pb²⁺ ions and L-cysteine.

Direct Formation of Colloidal All-Inorganic Metal Nanocrystals from Magic-Size Clusters

All-inorganic metal nanocrystals (NCs) offer an ideal platform for atomically precise surface design and show improved electrocatalytic activity compared to conventional NCs capped with organic ligands. Here, we show the possibility of obtaining colloidal all-inorganic Au NCs directly from magic-size clusters (MSCs) with the assistance of inorganic molecules, NOBF4 or Na2S2O8. The unique advantages of NOBF4 or Na2S2O8 as both oxidizing agents and stripping ligands are taken to tune the surface state of Au25(PET)18-TOA+ MSCs and assemble them to form either positively or negatively charged all-inorganic Au NCs. We show that positively charged all-inorganic Au NCs can be further modified with different functional groups, which provide the possibility to meet the target requirements. We found that the negatively charged NCs exhibit improved faradaic efficiency (FE = 92%) for the reduction of CO2 to CO at -0.369 V (vs RHE) and a 5-fold increase in current density compared to organic-capped Au NCs (FE = 67%). In addition, we extended this approach to other MSCs and formed all-inorganic metal NCs with different compositions and morphologies. The use of simple inorganic ligands to induce the conversion from MSCs to metal NCs enriches the current solution process of synthesizing all-inorganic NCs and can open up more opportunities for designing colloidal nanocrystal catalysts.

Efficient FAPbI3–PbS quantum dot graphene-based phototransistors

PbS quantum dots capped with formamidinium ligands were deposited as graphene-based photodetectors. Solid phase exchange improves the infrared photo-detectivity.

Thiol-Anchored TIPS-Tetracene Ligands with Quantitative Triplet Energy Transfer to PbS Quantum Dots and Improved Thermal Stability

Triplet energy transfer between inorganic quantum dots (QDs) and organic materials plays a fundamental role in many optoelectronic applications based on these nanocomposites. Attaching organic molecules to the QD as transmitter ligands has been shown to facilitate transfer both to and from QDs. Here we show that the often disregarded thiol anchoring group can achieve quantitative triplet energy transfer yields in a PbS QD system with 6,11-bis[(triisopropylsilyl)ethynyl]tetracene-2-methylthiol (TET-SH) ligands. We demonstrate efficient triplet transfer in a singlet fission-based photon multiplication system with 5,12-bis[(triisopropylsilyl)ethynyl]tetracene generating triplets in solution that transfer to the PbS QDs via the thiol ligand TET-SH. Importantly, we demonstrate the increased thermal stability of the PbS/TET-SH system, compared to the traditional carboxylic acid counterpart, allowing for higher photoluminescence quantum yields.

Solution Processing and Self-Organization of PbS Quantum Dots Passivated with Formamidinium Lead Iodide (FAPbI3)

Solution-processed lead sulfide quantum dots (PbS QDs) are very attractive as NIR-active semiconductors for the fabrication of cost-efficient optoelectronic devices. To control the thin film carrier transport, as well as stability, surface passivation is of crucial importance. Here, we present the successful surface passivation of PbS QDs by the formamidinium lead iodide (FAPbI3) ligand. An effective procedure for the fabrication of FAPbI3-passivated PbS QDs through a binary-phase ligand exchange protocol in hexane and n-methylformamide is demonstrated. It is shown that this solution-processed ligand exchange drastically changes the photoluminescence intensity, exciton recombination dynamics, and carrier lifetime of the nanocrystals. The solution casting of the ligand-exchanged nanocrystals into thin films results in the periodic ordering of QDs in a square superlattice with close contacts. Planar graphene/QD photodetectors fabricated with PbS QDs passivated with FAPbI3 show substantially increased thermal stability as compared to similar devices using PbS QDs passivated with commonly used methylammonium lead iodide.

Zero reduction luminescence of aqueous-phase alloy core/shell quantum dots via rapid ambient-condition ligand exchange

Quantum dots (QDs) have been widely studied as promising materials for various applications because of their outstanding photoluminescence (PL). Although ligand exchange methods for QDs have been developed over two decades, the PL quantum yield (QY) of aqueous phase QDs is still lower than that of their organic phase and the mechanism of quenching has not been clearly understood. In this study, we demonstrate for the first time that 3-mercaptopropionic-capped CdZnSeS/ZnS core/shell QDs obtained via ligand exchange in a ternary solvent system containing chloroform/water/dimethyl sulfoxide can enable the fast phase transfer and zero reduction of PL under ambient condition. The new solvent system allows the ligand-exchanged QDs to exhibit enhanced QYs up to 8.1% of that of the organic-phase QDs. Based on both theoretical calculation and experiment, it was found that control over the physical/chemical perturbation between the organic/aqueous phases by choosing appropriate solvents for the ligand exchange process is very important to preserve the optical properties of QDs. We believe that our new technologies and theoretical knowledge offer opportunities for the future design and optimization of highly stable and highly luminescent aqueous-phase QDs for various applications.

Characterization of the Ligand Exchange Reactions on CdSe/ZnS QDs by Capillary Electrophoresis

The continuous development of semiconductor quantum dots (QDs) in biochemical research has attracted special attention, and surface functionalizing becomes more important to optimize their performance. Ligand exchange reactions are commonly used to modify the surface of QDs for their biomedical applications. However, the kinetics of ligand exchange for semiconductor QDs remain fully unexplored. Here, we describe a simple and rapid method to characterize the ligand exchange reactions on CdSe/ZnS QDs by capillary electrophoresis (CE). The results of ultraviolet-visible absorption spectra, fluorescence spectra, and Fourier transform infrared spectroscopy indicated the successful implementation of the ligand exchange process. The dynamics of ligand exchange of OA-coated CdSe/ZnS QDs with 4-mercaptobenzoic acid was monitored by CE, and the observed ligand exchange trends were fitted with logistic functions. When the ligand exchange reactions reached equilibrium, the ligand density of QDs can be quantified by CE. It is anticipated that CE will be a new powerful technique for quantitative analysis of the ligand exchange reactions on the surface of QDs.

Near-Infrared Active Lead Chalcogenide Quantum Dots: Preparation, Post-Synthesis Ligand Exchange, and Applications in Solar Cells

Quantum dots (QDs) of lead chalcogenides (e.g. PbS, PbSe, and PbTe) are attractive near-infrared (NIR) active materials that show great potential in a wide range of applications, such as, photovoltaics (PV), optoelectronics, sensors, and bio-electronics. The surface ligand plays an essential role in the production of QDs, post-synthesis modification, and their integration to practical applications. Therefore, it is critically important that the influence of surface ligands on the synthesis and properties of QDs is well understood for their applications in various devices. In this Review we elaborate the application of colloidal synthesis techniques for the preparation of lead chalcogenide based QDs. We specifically focus on the influence of surface ligands on the synthesis of QDs and their solution-phase ligand exchange. Given the importance of lead chalcogenide QDs as potential light harvesters, we also pay particular attention to the current progress of these QDs in photovoltaic applications.

Exchange equilibria of carboxylate-terminated ligands at PbS nanocrystal surfaces

Exchange equilibria between carboxylic acids are quantified and ligand displacement mechanisms are probed on PbS nanocrystals synthesized via two different methods.