Oleic Acid Ligands Research Articles

Separating the Effects of Nonorthogonal Variables on the Hot-Injection Synthesis of Core/Thick-Shell (“Giant”) CdTe/CdS Quantum Dots

“Giant” core/thick-shell quantum dot (QD) nanostructures are of interest due to their unusual optical properties and importance as components of more advanced heterostructures tailored to achieve increasingly complex optical functions. However, reliable one-step seeded growth of these structures poses a significant challenge: one must balance multiple competing reaction processes to find the growth regime that realizes spherical shells of both the target size and high crystalline quality. Adjusting synthesis conditions in thicker-shelled reactions is further complicated by multiple nonorthogonal variables that impact the reaction mechanism. These variables include the reaction volume, reaction concentration, and oleic acid (ligand) concentration. Here, we investigate the seeded growth of core/thick-shell CdTe/CdS QDs by adapting a “flash” shelling method. We systematically vary three key reaction parameters (particle concentration, oleic acid:Cd ratio, and Cd–S:CdTe core ratio) over 30+ different thick-shelling reactions to elucidate the separate and intersecting impacts of these parameters on shell growth. Our analysis of the resulting particle quality reveals that the particle concentration of the reaction plays a critical role in the shell growth mechanism. We find the impact of oleic acid to be dependent on the particle concentration for a given shell thickness. We also find that the optimal conditions shift when targeting increasingly thick shells. The results demonstrate the importance of testing and controlling for synthesis variables across a multidimensional parameter space. We develop and present general experimental design criteria to help guide efficient development of new seeded growth reactions that enable reliable synthesis of thick-shelled nanostructures.

In Situ Bonding Regulation of Surface Ligands for Efficient and Stable FAPbI3 Quantum Dot Solar Cells.

Quantum dots (QDs) of formamidinium lead triiodide (FAPbI3 ) perovskite hold great potential, outperforming their inorganic counterparts in terms of phase stability and carrier lifetime, for high-performance solar cells. However, the highly dynamic nature of FAPbI3 QDs, which mainly originates from the proton exchange between oleic acid and oleylamine (OAm) surface ligands, is a key hurdle that impedes the fabrication of high-efficiency solar cells. To tackle such an issue, here, protonated-OAm in situ to strengthen the ligand binding at the surface of FAPbI3 QDs, which can effectively suppress the defect formation during QD synthesis and purification processes is selectively introduced. In addition, by forming a halide-rich surface environment, the ligand density in a broader range for FAPbI3 QDs without compromising their structural integrity, which significantly improves their optoelectronic properties can be modulated. As a result, the power conversion efficiency of FAPbI3 QD solar cells (QDSCs) is enhanced from 7.4% to 13.8%, a record for FAPbI3 QDSCs. Furthermore, the suppressed proton exchange and reduced surface defects in FAPbI3 QDs also enhance the stability of QDSCs, which retain 80% of the initial efficiency upon exposure to ambient air for 3000 hours.

Interfacial energy-level alignment via poly-3-hexylthiophene-CsPbI3 quantum dots hybrid hole conductor for efficient carbon-based CsPbI2Br solar cells

Carbon-based perovskite solar cells (C-PSCs) are promising contenders for the next generation solar cells with high stability and low cost. However, the absence of hole transport layer (HTL) results in a large energy-level mismatch at the interface between perovskite and carbon electrode, which hampers hole extraction and then leads to lower efficiency of C-PSCs compared with metal electrode-based PSCs. In this work, a novel hybrid hole conductor of poly-3-hexylthiophene - CsPbI3 quantum dots (P-QD) is used to improve hole extraction in CsPbI2Br C-PSCs. In this P-QD hybrid hole conductor, P3HT replaces the initial oleic acid ligands around QDs. This not only improves the stability but also facilitates charge extraction and transport of QDs. The application of P-QD in CsPbI2Br C-PSCs effectively improves hole extraction by aligning perovskite/carbon energy-level, thus reducing the non-radiative recombination loss in devices. Furthermore, due to the narrow bandgap, CsPbI3 QDs extend the optical response range of CsPbI2Br solar cells. As a result, the P-QD hybrid hole conductor increases the power conversion efficiency of CsPbI2Br C-PSCs from 13.49% to 15.04%. This strategy provides a new approach for the construction of HTL in C-PSCs.

Improved efficiency for lead-free CsCu2I3 halide microplatelets by suppressing surface pinholes via surface functionalization

Lead-free copper(I)-based halides have attracted tremendous attentions in producing optoelectronic devices due to their outstanding optical properties and low-cost synthesis. However, the low efficiency of copper(I)-based halides in the long-wavelength visible emissions (typically yellow- or red-light emissions) has hindered the use of such non-toxic halides in full-color displays and solid-state lighting applications. In this work, lead-free, air-stable and yellow-light emissive CsCu2I3 microplatelets (MPs) with well-defined smooth surface were synthesized by a one-pot anti-solvent solution method at room temperature via surface functionalization with different long-chain surfactants. The results suggested that oleic acid (OA) was the most favorable surfactant for the CsCu2I3 MPs with the best performance. Photoluminescence quantum efficiency of the yellow-emissive CsCu2I3 MPs (centered at 580 nm) was dramatically enhanced fourfold by coordinating OA ligands during the synthesis to eliminate the surface pinholes of the CsCu2I3 MPs. Notably, the CsCu2I3 MPs showed ultra-high stability towards exposure of ambient air over 50 days. These findings may open new directions to synthetize high-efficiency copper(I)-based halides that generate long-wavelength visible emissions for lead-free nontoxic full-color optoelectronic devices.

Solvent-mediated surface ligand exchange to enhance the performance of quantum-dot light-emitting diodes

Colloidal quantum dots (QDs) with core-shell structure and long-chain organic ligands are widely used as emitting-material layers (EMLs) for quantum-dot light-emitting diodes (QLEDs). However, the poor conductivity of long-chain organic ligands severely affects charge injection into QD-EMLs and charge transport among QDs, usually leading to poor device performance. To address the issue, we partially replaced the original long-chain oleic acid (OA) ligands capping on the QD surface with short-chain 1,2-ethanedithiol (EDT) ligands via solvent-mediated solid-state ligand exchange to fabricate high-performance QLEDs. The effects of solvent (acetonitrile and ethanol)-mediated EDT ligand exchange on the properties of QDs and device performance were comparatively investigated. Our results show that the ligand exchange is more sufficient when using ethanol instead of acetonitrile as the solvent for EDT, which is not only capable of improving charge transport among QDs but also capable of enhancing hole-electron injection balance in QLEDs. As a result, the QLED device based on QDs after EDT (in ethanol) ligand exchange exhibits a maximum luminescence of 190,200 cd/m 2 , a maximum current efficiency of 18.34 cd/A, a maximum power efficiency of 9.60 l m/W and a peak external quantum efficiency of 12.26%, appreciably outperforming those based on pristine QDs and QDs after EDT (in acetonitrile) ligand exchange. After aging, its maximum current efficiency, maximum power efficiency and peak external quantum efficiency can reach up to 35.91 cd/A, 23.30 l m/W and 24.00%, respectively. Our results highlight the importance of surface chemistry control via solvent-mediated ligand exchange to fabricate high-performance QLEDs, and suggest that ethanol may be a more suitable solvent for EDT to fulfill solid-state ligand exchange of core/shell QDs. • Solvent mediated 1,2-ethanedithiol (EDT) ligand exchange is reported to enhance the performance of QLEDs. • The ligand exchange is more sufficient when using ethanol instead of acetonitrile as the solvent for EDT. • Ethanol mediated ligand exchange can improve charge transport among QDs and enhance hole-electron injection balance in QLEDs. • Devices based on EDT/ethanol ligand-exchanged QDs surpass those based on pristine or EDT/acetonitrile ligand-exchanged QDs.

Investigation of Ag doping and ligand engineering on green synthesized CdS quantum dots for tuning their optical properties

We present a green route for the colloidal synthesis of undoped and silver (Ag) doped cadmium sulfide (CdS) quantum dots (QDs). We have used olive oil as the reaction medium, which acted as a source of oleic acid (OA) ligand in the green synthesis of CdS QDs. With the increase in OA concentration, the dispersibility of CdS QDs improved. The water-dispersible CdS QDs were prepared via exchanging OA’s associated ligand with 3-mercaptopropionic acid (MPA). The MPA-capped CdS QDs showed the disappearance of the S-H peak as characterized via FTIR. The crystal and optical properties of Ag-doped CdS QDs were investigated, and the spectral red shift in the absorption spectra was observed. The CdS QDs with low Ag doping concentration increased the lifetime of excitons, but the average lifetime was suppressed at a higher concentration. We also discussed the variation in the properties of the CdS QDs through ligand engineering and Ag doping. These doped and undoped QDs have the potential for applications in photocatalysis, water splitting, solar cells, etc. In addition, water dispersible QDs can be helpful for bioimaging, and drug delivery applications

Scalable synthesis of efficiently luminescent and color-tunable CsPbX3 (X=Cl, Br, I) nanocrystals by regulating the reaction parameters

All-inorganic CsPbX3 (X=Cl, Br, I) perovskite nanocrystals (NCs) exhibit attractive optoelectronic properties for lighting and displays application but suffer from the lack of profound insights into their reaction kinetics. Herein, a modified ligand-assisted reprecipitation (LARP) method was developed to synthesize high-quality CsPbBr3 NCs by utilizing oleic acid (OA) and n-octylamine (OTAm) as surface ligands, and the effect of Cs/Pb ratio, precursors and ligands concentration on the structure, morphology and fluorescence properties was systematically investigated. The monodisperse CsPbBr3 NCs with cubic shape and average particle size of 13.7 nm are obtained by optimizing the reaction parameters. Appropriately increasing the Cs/Pb ratio and precursors concentration to 0.5 and 1.0 mM are favorable for forming a more complete crystal structure with lower defect states density, which can significantly enhance the PL intensity. Notably, the surface defect states of CsPbBr3 NCs are effectively passivated by the coordinated OA and OTAm ligands, and the maximum photoluminescence quantum yield (PLQY) can reach to 85.2%. Importantly, the effective synthesis of high-quality CsPbBr3 NCs can be amplified up to 50 times of reaction volume after finely optimizing the reaction parameters. Based on a facile anion exchange process, the CsPbX3 NCs with variable halogen composition exhibit tunable emission of 443.3∼649.1 nm and a wide color gamut of 117% NTSC standard. This work provides profound sights on the reaction kinetics of perovskite NCs, and further promotes its practical application in optoelectronic field.

Controlling Cation Distribution and Morphology in Colloidal Zinc Ferrite Nanocrystals.

This paper describes the first synthetic method to achieve independent control over both the cation distribution (quantified by the inversion parameter x) and size of colloidal ZnFe2O4 nanocrystals. Use of a heterobimetallic triangular complex of formula ZnFe2(μ3-O)(μ2-O2CCF3)6(H2O)3 as a single-source precursor, solvothermal reaction conditions, absence of hydroxyl groups from the reaction solvent, and the presence of oleylamine are required to achieve well-defined, crystalline, and monodisperse ZnFe2O4 nanoparticles. The size of the ZnFe2O4 nanocrystals increases as the ratio of oleic acid and oleylamine ligands to precursor increases. The inversion parameter increases with increasing solubility of the precursor in the reaction solvent, with the presence of oleic acid in the reaction mixture, and with decreasing reaction temperature. These results are consistent with a mechanism in which ligand exchange between oleic acid and carboxylate ligands bound to the precursor complex influences the degree to which the reaction produces a kinetically trapped or thermodynamically stable cation distribution. Importantly, these results indicate that preservation of the triangular Zn–O–Fe2 core structure of the precursor in the reactive monomer species is crucial to the production of a phase-pure ZnFe2O4 product and to the ability to tune the cation distribution. Overall, these results demonstrate the advantages of using a single-source precursor and solvothermal reaction conditions to achieve synthetic control over the structure of ternary spinel ferrite nanocrystals.

Sensitivity improvement of hybrid active layer containing 2D nanoplatelets for indirect x-ray detector

In this paper, we attempted to improve the detection sensitivity of an indirect x-ray detector through using a hybrid active layer composed of a poly [N-90-heptadecanyl–2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) organic semiconductor and cadmium selenide nanoplatelets (CdSe NPLs) colloidal inorganic semiconductors. First, different blending ratio in the active layer (i.e. 2:1, 1:1, 1:2 and 1:3) of PCDTBT:CdSe NPL were examined, a sensitivity of 89.5 μC·Gyair −1·cm−2 was achieved using a 1:1 ratio due to the low series resistance (RS ) and defect density in this configuration. Then, the oleic acid (OA) that was initially applied in the CdSe NPL surface was replaced with pyridine ligands, this was done because the pyridine ligand is a short-chain ligand that can help charge transfer by reducing the distance between NPLs in the active layer. In addition, an experiment was conducted to determine the optimal ligand exchange time. A detector with an PCDTBT:CdSe NPL active layer fabricated using pyridine ligand exchange achieved a sensitivity of 219.8 μC·Gyair −1·cm−2 after an exchange time of 12 h, this is an improvement of 155% compared to the detector using a PCDTBT:CdSe NPL with the original OA ligands. Lastly, the optimal thickness for the PCDTBT:CdSe NPL active layer was investigated. The highest mobility of 7.60 × 10− 6 cm2/V·s was recorded after fabricating the layer using spin-coating at 1900 rpm, the highest sensitivity of 314.0 μC·Gyair −1·cm−2 was also achieved under these conditions. Compared to the initial state of the detector, our modifications improved the sensitivity of the PCDTBT:CdSe NPL detector by 251%.

Elucidating the Role of Antisolvents on the Surface Chemistry and Optoelectronic Properties of CsPbBrxI3-x Perovskite Nanocrystals.

Colloidal lead-halide perovskite nanocrystals (LHP NCs) have emerged over the past decade as leading candidates for efficient next-generation optoelectronic devices, but their properties and performance critically depend on how they are purified. While antisolvents are widely used for purification, a detailed understanding of how the polarity of the antisolvent influences the surface chemistry and composition of the NCs is missing in the field. Here, we fill this knowledge gap by studying the surface chemistry of purified CsPbBrxI3-x NCs as the model system, which in itself is considered a promising candidate for pure-red light-emitting diodes and top-cells for tandem photovoltaics. Interestingly, we find that as the polarity of the antisolvent increases (from methyl acetate to acetone to butanol), there is a blueshift in the photoluminescence (PL) peak of the NCs along with a decrease in PL quantum yield (PLQY). Through transmission electron microscopy and X-ray photoemission spectroscopy measurements, we find that these changes in PL properties arise from antisolvent-induced iodide removal, which leads to a change in halide composition and, thus, the bandgap. Using detailed nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR) measurements along with density functional theory calculations, we propose that more polar antisolvents favor the detachment of the oleic acid and oleylamine ligands, which undergo amide condensation reactions, leading to the removal of iodide anions from the NC surface bound to these ligands. This work shows that careful selection of low-polarity antisolvents is a critical part of designing the synthesis of NCs to achieve high PLQYs with minimal defect-mediated phase segregation.

Enhanced Luminance of CdSe/ZnS Quantum Dots Light-Emitting Diodes Using ZnO-Oleic Acid/ZnO Quantum Dots Double Electron Transport Layer.

The widely used ZnO quantum dots (QDs) as an electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) have one drawback. That the balancing of electrons and holes has not been effectively exploited due to the low hole blocking potential difference between the valence band (VB) (6.38 eV) of ZnO ETL and (6.3 eV) of CdSe/ZnS QDs. In this study, ZnO QDs chemically reacted with capping ligands of oleic acid (OA) to decrease the work function of 3.15 eV for ZnO QDs to 2.72~3.08 eV for the ZnO-OA QDs due to the charge transfer from ZnO to OA ligands and improve the efficiency for hole blocking as the VB was increased up to 7.22~7.23 eV. Compared to the QLEDs with a single ZnO QDs ETL, the ZnO-OA/ZnO QDs double ETLs optimize the energy level alignment between ZnO QDs and CdSe/ZnS QDs but also make the surface roughness of ZnO QDs smoother. The optimized glass/ITO/PEDOT:PSS/PVK//CdSe/ZnS//ZnO-OA/ZnO/Ag QLEDs enhances the maximum luminance by 5~9% and current efficiency by 16~35% over the QLEDs with a single ZnO QDs ETL, which can be explained in terms of trap-charge limited current (TCLC) and the Fowler-Nordheim (F-N) tunneling conduction mechanism.

Spin‐ and Stress‐Depending Electrical Transport in Nanoparticle Supercrystals: Sensing Elastic Properties of Organic Tunnel Barriers via Tunneling Magnetoresistance

AbstractThe spin‐dependent electrical transport in rigid inorganic‐inorganic layered systems is extensively applied for the detection of magnetic fields in data storage. In this work, spin‐dependent electrical transport in flexible organic‐inorganic supercrystals based on superparamagnetic iron oxide nanoparticles is investigated. These nanoparticles are stabilized by oleic acid ligands, which in turn are serving as tunneling barriers between individual magnetic nanoparticles. The resulting tunneling magnetoresistance (TMR) is tunable due to the elastic properties of these organic barriers. Applying external mechanical stress on this composite material will change the average distance between adjacent nanoparticles and will hence determine the resulting TMR‐effect amplitude. Thus, measured stress‐induced changes in the barrier thickness at sub‐nanometer scale allow for determining the mechanical properties of organic barrier molecules in the confined space between the particles. These results provide the foundation for a new type of mechanical sensor.

Synthetic Mechanisms in the Formation of SnTe Nanocrystals.

Infrared active colloidal semiconducting nanocrystals (NCs) are important for applications including photodetectors and photovoltaics. While much research has been conducted on nanocrystalline materials such as the Pb and Hg chalcogenides, less toxic alternatives such as SnTe have been far less explored. Previous synthetic work on SnTe NCs have characterized photophysical properties of the nanoparticles. This study focuses on understanding the fundamental chemical mechanisms involved in SnTe NC formation, with the aim to improve synthetic outcomes. The solvent oleylamine, common to all SnTe syntheses, is found to form a highly reactive, heteroleptic Sn-oleylamine precursor that is the primary molecular Sn species initiating NC formation and growth. Further, the capping ligand oleic acid (OA) reacts with this amine to produce tin oxide (SnOx), facilitating the formation of an NC SnOx shell. Therefore, the use of OA during synthesis is counterproductive to the formation of stoichiometric SnTe nanoparticles. The knowledge of chemical reaction mechanisms creates a foundation for the production of high-quality, unoxidized, and stoichiometric SnTe NCs.

Temperature dependent local structure coherence of surface-modified BaTiO3 nanocubes

BaTiO3 nanocubes capped by polar tetrafluoroborate (BF4−) ligands are shown to have enhanced ferroelectric order and undergo sharper ferroelectric to paraelectric phase transitions relative to nanocubes capped with nonpolar oleic acid (OA) ligands.

Highly efficient emission and high-CRI warm white light-emitting diodes from ligand-modified CsPbBr<sub>3</sub> quantum dots

All-inorganic CsPbBr₃ perovskite quantum dots (QDs) have received great attention in white light emission because of their outstanding properties. However, their practical application is hindered by poor stability. Herein, we propose a simple strategy to synthesize excellent stability and efficient emission of CsPbBr₃ QDs by using 2-hexyldecanoic acid (DA) as a ligand to replace the regular oleic acid (OA) ligand. Thanks to the strong binding energy between DA ligand and QDs, the modified QDs not only show a high photoluminescence quantum yield (PLQY) of 96% but also exhibit high stability against ethanol and water. Thereby warm white light-emitting diodes (WLEDs) are constructed by combining ligand modified CsPbBr₃ QDs with red AgInZnS QDs on blue emitting InGaN chips, exhibiting a color rendering index of 93, a power efficiency of 64.8 lm/W, a CIE coordinate of (0.44, 0.42) and correlated color temperature value of 3018 K. In addition, WLEDs based on ligand modified CsPbBr₃ QDs also exhibit better thermal performance than that of WLEDs based on the regular CsPbBr₃ QDs. The combination of improved efficiency and better thermal stability with high color quality indicates that the modified CsPbBr₃ QDs are ideal for WLEDs application.

Insights into the Structure and Self‐Assembly of Organic‐Semiconductor/Quantum‐Dot Blends

AbstractControlling the dispersibility of crystalline inorganic quantum dots (QD) within organic‐QD nanocomposite films is critical for a wide range of optoelectronic devices. A promising way to control nanoscale structure in these nanocomposites is via the use of appropriate organic ligands on the QD, which help to compatibilize them with the organic host, both electronically and structurally. Here, using combined small‐angle X‐ray and neutron scattering, the authors demonstrate and quantify the incorporation of such a compatibilizing, electronically active, organic semiconductor ligand species into the native oleic acid ligand envelope of lead sulphide, QDs, and how this ligand loading may be easily controlled. Further more, in situ grazing incidence wide/small angle X‐ray scattering demonstrate how QD ligand surface chemistry has a pronounced effect on the self‐assembly of the nanocomposite film in terms of both small‐molecule crystallization and QD dispersion versus ordering/aggregation. The approach demonstrated here shows the important role which the degree of incorporation of an active ligand, closely related in chemical structure to the host small‐molecule organic matrix, plays in both the self‐assembly of the QD and small‐molecule components and in determining the final optoelectronic properties of the system.

Self-assembly, stability, and photoresponse of PbS quantum dot films capped with mixed halide perovskite ligands

The type of passivating ligands and the ligand exchange method influence the quality of lead sulfide quantum dot films. This imparts on the efficiency of optoelectronic devices. To get a compact arrangement of the nanocrystals in a thin film (⁓100 nm) via self-assembling, we used organic-inorganic perovskites with mixed halides for the solid-state exchange of oleic acid ligands on PbS QDs (⁓ 4 nm). Formamidinium lead halides FAPbIxBr3-x (x= 3,2,1,0) were used. X-ray spectroscopy shows that successful replacement of oleic acid with FA happens by short immersion of the films (2 min) in the solution. Transmission electron microscopy shows that nano-scale cracks, short-range ordering, and fusion of the nanocrystals happen during the exchange with FAPbI3. Gradual substitution of I− with Br− ions improves the film homogeneity with fewer defects (nano-scale holes and cracks). Uniform cubic assembly of the nanocrystals without defects and high coverage density is obtained with FAPbBr3. The findings are applied to the fabrication of field effect phototransistors on Si/SiO2/graphene wafers. Photoresponse of the devices indicates that the stability of the films in humidified air (26 °C and 51% humidity) is improved from less than 14 days for iodine to more than 60 days for bromine. The ILIGHT/IDARK ratio, responsivity, and specific detectivity increase by 64%, 55%, and 60%, respectively, with FAPbBr3. Prevention of lead sulfide nanocrystal aggregation and the formation of large-scale self-assemblies with perovskite ligands is of technical importance for next-generation optoelectronic applications.

Surface Modification and Stabilization of Photoluminescence Perovskite Nanocrystals Via Atomic Layer Deposition

Photoluminescence perovskite nanocrystals (NCs) have shown significant potential in optoelectronic applications in view of their narrow band emission with high photoluminescence quantum yields (PLQYs) and color tunability. However, their poor stability in light, heat and water environments still hinders practical applications in optoelectronic and bioimaging fields due to their ionic character.Atomic layer deposition (ALD) has been developed as an attractive method to stabilize the crystal structure of perovskite NCs through encapsulation and surface passivation. In this presentation, several stabilization methods through ALD are introduced. First, a low-temperature Al2O3 ALD process was developed to enhance the stability of CsPbBr3 quantum dots-silica sphere in light, water and heat, which originated from the crystal structure stabilization after ALD coating. Nonetheless, a significant photoluminescence (PL) quenching of NCs was typically observed upon Al2O3 ALD. Accordingly, a specially designed ALD reactor integrated a FTIR spectrometer was exploited, which enabled in-situ characterizations to investigate ligands exchange and evolution during deposition after each precursor dosing. It was found that the surface chemical reaction between ALD precursor and capping oleic acid (OA) ligands led to reorganization of OA ligands that increased surface trap sites, leading to PL quenching. Based on the reaction mechanisms observed, a hybrid passivation strategy was developed to simultaneously enhance the photoluminescence quantum yield and the stability of perovskite NCs by two-step modification with surface halogen replenishment and ALD. Consequently, the PL quenching was avoided and the perovskite NCs/Al2O3 nanocomposites exhibited exceptional stability against water, light and heat. Our work provides a versatile method for preparing ultrastable perovskite NCs through ALD method and significantly improves their potential in LED illumination and backlight displays.

Ligands Anchoring Stabilizes Metal Halide Perovskite Nanocrystals

AbstractInstability of colloidal iodine‐based inorganic perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (IPNCs) represents a major obstacle in lead‐halide IPNC research and application. Herein, a ligand‐anchoring process is reported that enables significantly improved colloidal stability of the iodine‐based IPNCs for over 10 months in ambient. Apart from the previous efforts in searching for strong binding ligands to cap the IPNCs to incrementally reduce the exposure of the IPNC surface to the harsh colloidal environment, the ligand‐anchoring method demonstrates that such an exposure can be reduced substantially by suppressing the dynamic ligand exchange around the colloidal IPNCs. In the IPNC synthesis solution with common oleic acid (OA) and oleylamine (OLA) ligands with relative weak binding to IPNCs, a systematic reduction of the ligand concentration using hexane by an order of magnitude has shown to be effective in achieving OA/OLA ligand‐anchored iodine‐based IPNCs with superior stability as confirmed in optical absorption, photoluminescence, 1H solution nuclear magnetic resonance spectroscopy, and photoresponse. This result has revealed that the intermittent exposure of the IPNC surface during the dynamic ligand exchange is a primary mechanism underlying the colloidal IPNC instability, which can be resolved in the ligand‐anchoring process by suppressing such dynamic activities.

Constitutive and fracture behavior of ultra-strong supercrystalline nanocomposites

Supercrystalline nanocomposites are a new class of hybrid and nanostructured materials that can reach exceptional mechanical strength and can be fabricated at low temperatures. Hierarchically arranged, they bridge the gap from the nano- to the macro-scale. Even though their mechanical properties are starting to be characterized, their constitutive behavior is still largely unexplored. Here, the mechanical behavior of supercrystalline nanocomposites of iron oxide nanoparticles, surface-functionalized with oleic acid and oleyl phosphate ligands, is investigated in both bending and compression, with loading–unloading tests. A new bar geometry is implemented to better detect deformation prior to unstable crack propagation, and notched bending bars are tested to evaluate fracture toughness. Micro-mechanical tests result in the values of strength and elastic modulus that are extremely high for supercrystals, reaching record-high numbers in the oleic acid-based nanocomposites, which also show a significant tension–compression asymmetry. The constitutive behavior of both materials is predominantly linear elastic, with some more marked nonlinearities arising in the oleyl phosphate-based nanocomposites. The fracture toughness of both types of nanocomposites, ∼0.3 MPa√m, suggests that extrinsic toughening, associated with both material composition and nanostructure, plays an important role. Fractographic observations reveal analogies with shear and cleavage in atomic crystals. The influence of material composition, nanostructure, and processing method on the mechanical behavior of the nanocomposites is analyzed.