Oleic Acid Ligands Research Articles

Electron-Induced Degradation in Blue Quantum-Dot Light-Emitting Diodes.

The poor stability of blue quantum-dot (QD) light-emitting diodes (B-QLEDs) hinders their application in displays. To improve the stability of B-QLEDs, the degradation mechanism should be revealed. Here, the degradation mechanism of B-QLEDs is investigated by monitoring the changes occurring in the QDs and the hole transport layers (HTL) during device operation, respectively. It is revealed that the accumulation of electrons within the QDs is responsible for the degradation of the devices. On the one hand, the accumulated electrons induce the detachment of oleic acid ligands, leading to permanent damage to the stability of B-QDs. On the other hand, the accumulated electrons leak into the HTL or recombine at the HTL/QDs interface, leading to the degradation of HTL. The formation of surface defects in B-QDs and the decomposition of HTL contribute to the degradation of B-QLEDs. The results reveal the strong dependence of B-QLEDs stability on the accumulated electrons, the QDs and the HTL, which can help researchers to develop effective design strategies for improving the lifespan of B-QLEDs.

Versatile Organogels of Aluminum Oxide Subnanosheets for Locking Solvents and Adhesion.

Herein, we present a facile strategy to prepare versatile aluminum oxide subnanometer nanosheets with oleic acid and stearic acid ligands (OA-Al SNSs and SA-Al SNSs, respectively). The size effect endows subnanosheets with abundant acting sites, remarkable intermolecular interactions, and unique polymer-like properties, including flexibility, viscoelasticity, and sol-gel transitions, which is quite different from traditional inorganic materials. Consequently, subnanosheets could form freestanding organogels and OA-Al SNSs exhibit satisfying semisolidification of various solvents, making it an intriguing candidate for the safe storage and transportation of solvents. Furthermore, SA-Al SNSs exhibit excellent adhesive properties of high strength on diverse substrates, and it is easy to erase it without any damage, demonstrating the promising prospects in practical applications.

The Impact of Ligand Removal on the Optoelectronic Properties of Inorganic and Hybrid Lead Halide Perovskite Nanocrystal Films

AbstractLigand exchange performed during or after the colloidal synthesis of nanocrystals (NCs) provides an efficient way to produce conductive NC solids for optoelectronics. Herein, a post‐synthetic ligand washing process is developed and applied to two different combinations of ligands and perovskite NCs, namely robust green CsPbBr3 NCs capped by didodecyldimethylammonium bromide and near‐infrared FAPbI3 NCs decorated by weakly bound oleic acid ligands. The impact of such processes on the morphological and optoelectronic NC properties is examined while exploring parameters such as the reaction time and the influence of oxygen and humidity. For the FAPbI3 NCs, ligand washing results in extended NC aggregation and substantial photoluminescence loss, with the treatment becoming more aggressive for air‐exposed films. For the CsPbBr3 NCs, the process is insensitive to the environmental conditions and results in partial ligand shell loss and NC close packing rather than bulk‐like aggregation while affecting less the optical properties. Upon ligand removal, the photoconductance increases by up to ≈90% and ≈60% for FAPbI3 NCs and CsPbBr3 NCs, respectively. THz spectroscopy produces qualitatively similar trends of the conductivity with ligand removal time, with THz mobility values as high as 30 and 6 V−1s−1cm2 for glove box prepared FAPbI3 and CsPbBr3 NCs, respectively.

Nanoplatelet Superlattices by Tin-Induced Transformation of FAPbI3 Nanocrystals.

The transition from 3D to 2D lead halide perovskites is traditionally led by the lattice incorporation of bulky organic cations. However, the transformation into a coveted 2D superlattice-like structure by cationic substitution at the Pb2+ site of 3D perovskite is unfamiliar. It is demonstrated that the gradual increment of [Sn2+ ] alters the FASnx Pb1- x I3 nanocrystals into the Ruddlesden-Popper-like nanoplatelets (NPLs), with surface-absorbed oleic acid (OA) and oleylamine (OAm) spacer ligand at 80°C (FA+ : formamidinium cation). These NPLs are stacked either by a perfect alignment to form the superlattice or by offsetting the NPL edges because of their lateral displacements. The phase transition occurs from the Sn/Pb ratio ≥0.011, with 0.64 wt% of Sn2+ species. At and above Sn/Pb = 0.022, the NPL superlattice stacks start to grow along [00l] with a repeating length of 4.37(3) nm, comprising the organic bilayer and the inorganic block having two octahedral layers (n = 2). Besides, a photoluminescence quantum yield of 98.4% is obtained with Sn/Pb = 0.011 (n ≥ 4), after surface passivation by trioctylphosphine (TOP).

Trade‐Off Between Efficiency and Stability of CsPbBr3 Perovskite Quantum Dot‐Based Light‐Emitting Diodes by Optimized Passivation Ligands for Br/Pb

AbstractAlthough significant progress has been made in improving the external quantum efficiencies (EQEs) of perovskite quantum dot (QD) light‐emitting diodes (QLEDs), understanding the degradation mechanism and enhancing stability remain a challenge. Herein, increasing the content of Br‐based passivation ligands is shown to enhance the EQE up to 16.1% by reducing the defects of CsPbBr3 QDs in a Br‐rich environment. However, the operational lifetimes of perovskite QLEDs gradually decrease with the increase of halide content, owing to the intensified ion migration under continuous electric field confirmed by the current behavior of QLEDs and time‐of‐flight secondary‐ion mass spectrometry. Furthermore, a thorough analysis of the relationship between electricity and luminance of QLEDs suggests that a small amount of residue oleic acid ligands could weaken ion migration. Accordingly, a halide‐ and acid‐hybrid (HAH) co‐passivation strategy is proposed to optimize the content of Br‐ and acid‐based ligands, and achieve a maximum EQE of 18.6% and an operational lifetime (T50, extrapolated) of 213 h for CsPbBr3 QLEDs. This approach for passivating QDs combines the high efficiency of Br‐based ligands with the improved stability of acid‐based ligands. The study elucidates the correlation between ligands and device performance, highlighting the significance of two or even multiple ligands for efficient and stable perovskite QLEDs.

Ultra-rapid transfer of oleic acid-coated hydroxyapatite nanocrystals into water via a sodium citrate-assisted ligand exchange strategy

Precise control of the size, morphology and surface hydrophilicity of HA nanocrystals is an important prerequisite for the realization of relevant applications, especially biologically relevant ones. Although sodium oleate-assisted hydrothermal methods have received considerable attention due to their impressive synthesis quality and extremely wide aspect ratio tunability. However, the synthesized nanocrystals are usually hydrophobic due to oleic acid encapsulation, which greatly limits their wide application in biomedical fields. In view of this, we have established a robust ligand exchange strategy with sodium citrate as the exchange ligand and a mixture of water and ethanol as the exchange environment. The ligand exchange reaction requires only 5 min of stirring at 25 °C. DLS and zeta potential results show that the ligand-exchanged HA nanocrystals are highly negatively charged in water and dispersed as individual particles in water. TG and IR results showed that the oleic acid ligands on the surface of the nanocrystals were completely replaced by citric acid ligands. TEM images showed no change in the size and morphology of the nanocrystals before and after ligand exchange. Moreover, the colloidal stability of the ligand-exchanged HA nanocrystals in water could be quickly recovered in a few seconds even after freeze-drying into powder. The success of ligand exchange is mainly attributed to the fact that sodium citrate complexes calcium ions more strongly than sodium oleate, as well as the addition of ethanol eliminates the oil-water interface providing the basis for the approach of sodium citrate molecules to HA nanocrystals. This strategy undoubtedly bridges the gap that oleic acid-coated HA nanocrystals can be quickly and efficiently transferred to water, allowing HA nanocrystals to be used in bio-related applications without any obstacles.

All-Inorganic CsPbBr3 Perovskite Nanocrystals Synthesized with Olive Oil and Oleylamine at Room Temperature.

Room temperature (RT) synthesis of the ternary cesium lead bromide CsPbBr3 quantum dots with oleic acid and oleylamine ligands was developed by Zeng and coworkers in 2016. In their works, the supersaturated recrystallization (SR) was adopted as a processing method without requiring inert gas and high-temperature injection. However, the oleic acid ligand for haloplumbate is known to be relatively unstable. Hence, in this work, we employed the eco-friendly olive oil to replace the oleic acid portion for the SR process at RT. Resultantly, we found that the cube-shaped nanocrystal has a size of ~40-42 nm and an optical bandgap of ~2.3 eV independent of the surface ligands, but the photoluminescence lifetime (τav) and crystal packing are dependent on the ligand species, e.g., τav = 3.228 ns (olive oil and oleylamine; here less ordered) vs. 1.167 ns (oleic acid and oleylamine). Importantly, we explain the SR mechanism from the viewpoint of the classical LaMer model combined with the solvent engineering technique in details.

Magnetic heating of water dispersible and size-controlled superparamagnetic cobalt iron oxide nanoparticles

Magnetic nanoparticles have been of great interest for various biomedical applications due to their controllability in the distance. Monodisperse superparamagnetic CoFe2O4 (CFO) nanoparticles were synthesized by solvothermal method with oleic acid. The oleic acid ligands are exchanged with citric acid to obtain water-dispersible citric acid-coated CA (CFO–CA) nanoparticles. The M–H curve recorded at room temperature shows the superparamagnetic nature of CFO nanoparticles for both oleic- and citric-coated samples. The magnetic heating performance of CFO–CA shows an exciting dependence on the particle size and surface functionality. The hyperthermia efficiency of CFO–CA nanoparticles increases with increasing magnetic field strength and nanoparticle concentration. A specific absorption ratio value of 152.7 W/g obtained in this study is comparable to other investigations, demonstrating a great potential of CFO–CA nanoparticles for hyperthermia-based treatments. This controlled synthesis process can be applied to various functional biomedical nanoparticles to prepare nanoparticles with designed surface moieties for further potential applications.

Effect of Acidic Strength of Surface Ligands on the Carrier Relaxation Dynamics of Hybrid Perovskite Nanocrystals.

Perovskite nanocrystals (PeNCs) are known for their use in numerous optoelectronic applications. Surface ligands are critical for passivating surface defects to enhance the charge transport and photoluminescence quantum yields of the PeNCs. Herein, we investigated the dual functional abilities of bulky cyclic organic ammonium cations as surface-passivating agents and charge scavengers to overcome the lability and insulating nature of conventional long-chain type oleyl amine and oleic acid ligands. Here, red-emitting hybrid PeNCs of the composition CsxFA(1-x)PbBryI(3-y) are chosen as the standard (Std) sample, where cyclohexylammonium (CHA), phenylethylammonium (PEA) and (trifuluoromethyl)benzylamonium (TFB) cations were chosen as the bifunctional surface-passivating ligands. Photoluminescence decay dynamics showed that the chosen cyclic ligands could successfully eliminate the shallow defect-mediated decay process. Further, femtosecond transient absorption spectral (TAS) studies uncovered the rapidly decaying non-radiative pathways; i.e., charge extraction (trapping) by the surface ligands. The charge extraction rates of the bulky cyclic organic ammonium cations were shown to depend on their acid dissociation constant (pKa) values and actinic excitation energies. Excitation wavelength-dependent TAS studies indicate that the exciton trapping rate is slower than the carrier trapping rate of these surface ligands.

Bridging Chloride Anions Enables Efficient and Stable InP Green Quantum‐Dot Light‐Emitting Diodes

AbstractZnMgO nanoparticles (NPs) are commonly used as the electron transport layer (ETL) in indium phosphide (InP) based quantum dots light‐emitting diodes (QLEDs). It has been experimentally found that the inherent oxygen vacancy defects in ZnMgO can be passivated by halogen additives. However, an in‐depth understanding of how the halogen additives in ZnMgO affect the quantum dots (QDs) films is currently missing. Here, the study reports on efficient and stable indium phosphide (InP) green QLEDs by effectively bridging QDs and ETL using chloride (Cl) ions. As bi‐functional anchoring additives, Cl ions not only passivate the oxygen vacancy defects of ZnMgO NPs for suppressing the exciton quenching at the QDs/ZnMgO interfaces, but also facilitate the hole transport of QDs due to part replacement of insulated oleic acid ligands on the surfaces of InP QDs with Cl anions for more balanced charge injection in the devices. Consequently, the optimized green InP QLED achieves a peak external quantum efficiency (EQE) of 13.8% and an operational lifetime of 5944 h, which, to the best of current knowledge, represents the best overall performance among the reported green InP QLEDs.

Rapid and one-step mechanochemical ligand exchange of Yb3+/Er3+ co-doped NaGdF4 upconversion nanoparticles for efficient MR and CT imaging

Abstract Upconversion nanoparticles possess the ability to convert low energy photons to high energy photons, with the advantages of no autofluorescence, reduced photodamage, deeper tissue penetration, and extended photostability. Therefore, upconversion nanoparticles are considered ideal probes for framing potential theranostic agents. Generally, oleic acid has been used as a capping agent to fabricate high-quality upconversion nanoparticles. However, the hydrophobic oleic acid ligands on the surface of the nanoparticles continue to be a barrier when used in biological applications. Herein, we modified the surface of oleic acid coated upconversion (OA-NaGdF4:Yb/Er) nanoparticles to be hydrophilic using a novel one-step solvent assisted mechanochemical (SAM) milling ligand exchange strategy. Normal ligand exchange processes are tedious and take one to two days to complete, but the SAM ligand exchange strategy presented here is facile, rapid, and takes less than 1 h. This surface modified citric acid coated upconversion (CA-NaGdF4:Yb/Er) nanoparticles further ensures dispersibility in water and good biocompatibility, as revealed by electron microscopy, ATR-FTIR spectroscopy, zeta potential measurement, upconversion luminescence studies, and cytotoxicity assessment. Besides, the size and shape of the nanoparticles were retained after surface modification. Moreover, the monodispersed CA-NaGdF4:Yb/Er nanoparticles exhibit intrinsic paramagnetic behavior which makes them suitable to be used as a contrast enhancer for T 1-weighted MRI. In addition, the measured CT numbers (in HU) increased linearly with increasing the concentration of the surface modified CA-NaGdF4:Yb/Er nanoparticles, indicating their plausibility as a CT contrast agent. The current findings suggest that the SAM ligand exchange strategy could be used to fabricate hydrophilic and biocompatible upconversion nanoparticles for bioimaging applications.

Suppressing Ion Migration of Mixed‐Halide Perovskite Quantum Dots for High Efficiency Pure‐Red Light‐Emitting Diodes

AbstractPerovskite‐based light‐emitting diodes (PeLEDs) with a mixed halide composition can be used to obtain the “pure red” emission, i.e., in the 620–650 nm range, required for high‐definition displays. However, fast halide ion migration induces phase separation in these materials under electric fields, resulting in poor spectral stability and low efficiency. Herein, a method for producing mixed halide CsPbI3‐xBrx quantum dots (QDs) is reported in which ion migration is suppressed. The mixed halide composition is first achieved by anion exchange between CsPbI3 QDs and hydrobromic acid (HBr), during that the bromine ions efficiently passivate the iodine vacancies of the QDs. The original oleic acid ligands are then exchanged for 1‐dodecanethiol (1‐DT), which suppresses halide ion migration via the strong binding of the sulfhydryl group with the QD surface. PeLEDs based on these QDs exhibit a pure‐red electroluminescence (EL) peak at 637 nm, a maximum external quantum efficiency (EQE) of 21.8% with an average value of 20.4%, a peak luminance of 2653 cd m−2, and low EQE decease with increasing luminance. The EL spectrum of these devices is stable even at 6.7 V and they have an EQE half‐life of 70 min at an initial luminance of 150 cd m−2.

Stabilization of PbS colloidal-quantum-dot gas sensors using atomic-ligand engineering

Colloidal quantum dots (CQDs) are promising materials candidates for next-generation semiconductor gas sensors, due to their high surface activity and solution-processability. Despite the improved sensitivity demonstrated by PbS CQDs, their long-term stability remains a challenge for practical applications. In this paper, we propose a stabilization strategy for PbS CQD gas sensors by using atomic-ligand engineering. The oleic acid ligands surrounding PbS CQDs are replaced by halide ligands (Cl–, Br– and I–, respectively) via a phase-transfer ligand exchange method. The sensitivity and long-term stability of the PbS CQD gas sensor towards nitrogen dioxide (NO2) at room temperature are impressively improved. The excellent humidity stability of the iodine ligand exchanged PbS CQD gas sensor is also demonstrated. The mechanism is investigated by real-time electrical measurement, combined with operando diffuse reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis. The results indicate that the large atomic radius and high binding energy of iodide ligands enhance surface passivation and provide favorable conditions for NO2 adsorption and charge transfer. This study highlights the importance of surface ligand engineering in the development of practical CQD gas sensors.

Enhanced Long‐Term Luminescent Stability through Near‐Single‐Dot Passivation and Encapsulation of Perovskite Quantum Dots for Printable Photonics

Metal halide perovskites quantum dots (QDs) stand at the forefront of multifarious photonic applications, including micro‐light‐emitting diode and further augmented reality, virtual reality, and other novel display, lighting technologies. Barriers to applications, however, lie in their toxicity of lead, instability to light, moisture and heat, and processability at the nanoscale‐particle level. Herein, a simple and versatile postprocessing approach is reported for the near‐single‐dot passivation and encapsulation of representative lead‐free double perovskite Cs2Ag0.4Na0.6InCl6:Bi through liquid‐phase processing of perhydropolysilazane and quantum dots colloid with controllable hydrolysis curing. The conventional unstable oleylamine and oleic acid ligands are replaced by ‐NCl bonding on the surface of nanocrystal, accompanied by the resulting compact and robust silica layer without compromising the optical properties of the quantum dots. With the near‐single‐dot protection, the quantum dots do not show fluorescence quenching even when stored for more than 90 days and exhibit remarkably improved stability against heat, ultraviolet irradiation and humidity compared to the raw quantum dots. The strategy offers a versatile way of creating nanoscale‐particle level protection of luminescent quantum dots, and can be universally compatible with solution‐based patterning techniques and photonics applications where quantum dots are used.

Re-Examination of the Polymer Encapsulation of Quantum Dots for Biological Applications

The application of semiconductor core–shell quantum dots in biology requires the dispersion of these nanoparticles in water. A widely used phase transfer strategy is their encapsulation with amphiphilic polymers, a method that is believed to preserve the original ligand shell of quantum dots. In this contribution, we test this belief for the first time using a common encapsulation protocol based on poly(styrene-co-maleic anhydride) and ethanolamine and demonstrate that the original quantum dot ligand shell is in fact altered during the polymer encapsulation. In particular, our results show that primary amines, which are typically added to increase the hydrophilicity of the encapsulating polymer, displace the original ligands as a necessary step in the phase transfer process. We demonstrate that the surface of quantum dots synthesized using phosphinic acid as a ligand is readily modified by ethanolamine and hence shows high phase transfer efficiency, whereas quantum dots synthesized using oleic acid ligands did not show reactivity toward the primary amine and failed to transfer in the same conditions. Instead, a thiol, mercaptoethanol, was employed to displace the original oleic acid ligands and promote efficient poly(styrene-co-maleic anhydride) encapsulation. This work also provides practical guidelines for how the quantum dot community can better design and implement phase transfer protocols for a given composition, in order to maximize the efficiency and durability of the resulting water-dispersible nanoparticles for biological applications.

High-Frequency EPR and ENDOR Spectroscopy of Mn2+ Ions in CdSe/CdMnS Nanoplatelets.

Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn2+ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn2+ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn2+ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn2+ ions. The interaction between surface Mn2+ ions and 1H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn2+ ions and 1H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn2+ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.

Suppression of blinking in single CsPbBr3 perovskite nanocrystals through surface ligand exchange.

Photoluminescence blinking in individual semiconducting and perovskite quantum dots reflects reduced emission quantum yield and represents an obstacle towards quantum dot applications. One of the origins of blinking is the presence of surface structural defects that can function as charge traps. To reduce the defects the surface can be modified by, e.g., covering with ligands that are more strongly bound to the surface. Here, we report exchange of ligands on the CsPbBr3 perovskite nanocrystal surface and the effect of the exchange on photoluminescence blinking. Replacement of the oleic acid and oleylamine ligands which are used in the synthesis process with quaternary amine ligands leads to substantial increase of photoluminescence quantum yield. On single particle level this is reflected by significantly improved blinking characteristics. Statistical analysis using the probability density function shows that the ligand exchange leads to longer duration of ON-times and shorter OFF-times, as well as to the presence of a higher fraction of ON-time intervals. These characteristics are not affected by sample aging within three weeks. On the contrary, storage of the samples in solution for one-to-two weeks leads to further improvement of the ON-time interval fraction statistics.

Controllable structural and optical properties of NaYF4:Tm, Yb microparticles by Yb3+ doping for anti-counterfeiting.

Hexagonal NaYF4:Tm, Yb upconversion (UC) phosphors with excellent UC luminescence quantum efficiency and chemical stability meet demands for applications in bioimaging and anti-counterfeiting printing. In this work, a series of NaYF4:Tm, Yb upconversion microparticles (UCMPs) with different concentrations of Yb were synthesized by a hydrothermal method. Then, the UCMPs become hydrophilic through surface oxidation of the oleic acid (C-18) ligand to azelaic acid (C-9) using the Lemieux-von Rodloff reagent. The structure and morphology of UCMPs were investigated by X-ray diffraction and scanning electron microscopy. The optical properties were studied using diffusion reflectance spectroscopy and photoluminescent spectroscopy under 980 nm laser irradiation. The emission peaks of the Tm3+ ions are 450, 474, 650, 690, and 800 nm, attributed to the transitions from the excited state to ground state 3H6. These emissions are the results of two or three photon absorption through multi-step resonance energy transfer from excited Yb3+, confirmed via a power-dependent luminescence study. The results show that the crystal phases and luminescence properties of the NaYF4:Tm, Yb UCMPs are controlled by changing the Yb doping concentration. The printed patterns are readable under the excitation of a 980 nm LED. Moreover, the zeta potential analysis shows that the UCMPs after surface oxidation are water dispersible. In particular, the naked eye can observe the enormous upconversion emissions in UCMPs. These findings indicated that this fluorescent material is an ideal candidate for anti-counterfeiting and biological applications.

Dual functional phosphate-ester BODIPY regulation achieved stable CsPbBr3 nanocrystals for optical anti-counterfeiting

A proton-free phosphate-ester BDPPO was used as a substitute for commonly used oleic acid ligands for the enhanced binding of ligands onto the surface of CsPbBr3 to synthesize stable CsPbBr3 for encryption and decryption of information.

Mixed halide perovskite nanocrystals with surface engineering based on pseudohalide passivation and Short-Chain ligand exchange for High-Performance blue Light-Emitting diodes

Perovskite nanocrystals (PeNCs) have attracted considerable interest in light-emitting diodes (LEDs) owing to their unique optoelectronic properties. Although significant progress has been made in green and red perovskite LEDs (PeLEDs) with PeNCs, the performance of blue PeLEDs is still limited. The main obstacles for blue PeLEDs based on PeNCs are their Cl vacancy-induced low quantum efficiency in mixed halide (Br/Cl) systems and high electrical resistance from long organic capping ligands. In this study, to overcome these issues and achieve high-performance blue PeLEDs, a surface engineering (SE) strategy based on pseudohalide defect passivation and short-chain ligand exchange was introduced. By passivating Cl vacancies with SCN– and exchanging long-chain oleic acid and oleylamine ligands with short octylphosphonic acid and 3,3-diphenylpropyl amine, respectively, the optoelectronic properties of the PeNCs were significantly enhanced. The blue PeLEDs based on PeNCs, using SE, exhibited a maximum luminance of 1874 cd/m2 and maximum external quantum efficiency of 4.9 % at an electroluminescence wavelength of 467 nm, which satisfies the blue primary color requirement (467 nm) in Rec. 2020 color space.